Plural of Thesis: Meaning, Rules, and Easy Examples

Understanding the plural of thesis can be tricky for learners of English. The word looks simple, but its plural form does not follow the normal rule of adding “s.” This is because the thesis comes from Greek, and words borrowed from ancient languages often keep irregular forms. In academic writing, you’ll often see a thesis used to describe a research project, and knowing how to use the correct plural is important for clarity.

A thesis is a statement or research project that explains an argument or idea. In schools and universities, a thesis is often a long paper written by students. The plural of thesis is thesis, and this spelling often confuses new writers. Unlike regular plurals, you cannot simply write “thesises.” The only correct plural form is theses, and this change follows a rule that many Greek-based words share.

Core Words

Word (Singular)	Plural Form	Meaning	Example Sentence
Thesis	Theses	A long research paper or statement of argument	The students defended their theses last week.
Analysis	Analyses	A detailed examination of something	The professor compared different analyses of the data.
Crisis	Crises	A difficult or dangerous situation	Several crises affected the country at once.
Basis	Bases	The main support or foundation	Their bases for the argument were weak.
Hypothesis	Hypotheses	A testable idea or explanation	The scientists proposed two hypotheses for the results.
Diagnosis	Diagnoses	Identification of a disease or problem	Doctors made quick diagnoses of the illness.
Parenthesis	Parentheses	Curved marks used in writing ( )	Use parentheses to add extra information.
Prognosis	Prognoses	Prediction of the future outcome	The doctors gave positive prognoses for recovery.
Synthesis	Syntheses	Combining elements to form something new	The book explains various syntheses of theories.
Ellipsis	Ellipses	Omission of words, often marked by (…)	Writers often use ellipses to show pauses.

Academic and Scientific Terms

Word (Singular)	Plural Form	Meaning	Example Sentence
Synopsis	Synopses	A short summary of a larger work	The teacher asked for quick synopses of each chapter.
Axis	Axes	A fixed line used for measurement or structure	The graph has two axes for data comparison.
Oasis	Oases	A fertile spot in the desert	Travelers rested at several oases on the journey.
Stimulus	Stimuli	Something that causes a response	The brain reacts to different stimuli quickly.
Curriculum	Curricula	A set of courses in a school or program	Universities update their curricula often.
Phenomenon	Phenomena	An observable fact or event	Natural phenomena attract scientists worldwide.
Medium	Media	A means of communication or material	Social media shape public opinion.
Datum	Data	A single piece of information	The research relies on accurate data.
Criterion	Criteria	A standard or rule for judgment	The judges set clear criteria for success.
Memorandum	Memoranda	A written note or record	Companies send out memoranda about policy changes.

Language and General Usage

Word (Singular)	Plural Form	Meaning	Example Sentence
Index	Indices	A list for quick reference or a math measure	Stock indices rose sharply this year.
Appendix	Appendices	Additional material at the end of a book	The author included charts in the appendices.
Vertex	Vertices	A meeting point of two lines or edges	The cube has eight vertices.
Matrix	Matrices	An arrangement of numbers in rows and columns	The students solved complex matrices in class.
Larva	Larvae	The young form of an insect before becoming an adult	The pond was full of mosquito larvae.
Alumnus	Alumni	A graduate of a school or university	The college invited its alumni to the reunion.
Radius	Radii	A line from the center to the edge of a circle	The teacher compared the radii of two circles.
Genus	Genera	A biological classification of organisms	Several genera belong to the same family.
Nucleus	Nuclei	The central part of an atom or cell	Scientists studied the cell nuclei under a microscope.
Fungus	Fungi	An organism like mold, yeast, or mushroom	The forest floor was covered in fungi.

Science and Medicine

Word (Singular)	Plural Form	Meaning	Example Sentence
Corpus	Corpora	A collection of written or spoken texts	Linguists study language using large corpora.
Bacterium	Bacteria	A microscopic single-celled organism	Harmful bacteria can cause infections.
Medium (art/material)	Media	A substance or material for growth or expression	The artist worked with different media.
Stratum	Strata	A layer of material or society	The archaeologists uncovered ancient strata.
Species	Species	A group of living organisms with similar traits	Many species are endangered today.
Axis (anatomy)	Axes	The central structure around which something turns	Doctors studied the spinal axes.
Formula	Formulae	A set of rules or a mathematical expression	Chemists used several formulae in their research.
Stimulus (psychology)	Stimuli	A thing that triggers a reaction	The child responded to bright stimuli.
Vertebra	Vertebrae	Bones that form the spine	The X-ray showed three damaged vertebrae.
Papilla	Papillae	A small rounded projection on the body	The tongue has many papillae for taste.

Advanced Academic and Biology

Word (Singular)	Plural Form	Meaning	Example Sentence
Phenomenon	Phenomena	An observable fact or event	Scientists studied natural phenomena like eclipses.
Criterion	Criteria	A standard for making a judgment	The job has strict criteria for selection.
Datum	Data	A single piece of information	The survey collected valuable data.
Curriculum	Curricula	A program of study in education	Schools update their curricula each year.
Medium (communication)	Media	Channels of communication	The media reported the event worldwide.
Ovum	Ova	A female reproductive cell	The doctor studied the patient’s ova.
Addendum	Addenda	An extra addition to a document	The contract included two addenda.
Memorandum	Memoranda	A written reminder or note	The manager sent memoranda to staff.
Bacterium (again, emphasis biology)	Bacteria	Microscopic single-celled organisms	Bacteria live in soil and water.
Stigma	Stigmata	Marks or signs with symbolic meaning	The monk was said to bear holy stigmata.

Grammar, Science, and Classical Roots

Word (Singular)	Plural Form	Meaning	Example Sentence
Appendix (anatomy)	Appendices	A small organ attached to the intestine	Surgeons removed patients’ infected appendices.
Index (books/math)	Indices	A list or measure used for reference	Economic indices show growth and decline.
Codex	Codices	An ancient manuscript in book form	Historians studied old codices in the library.
Locus	Loci	A specific position or location	Genes are found at different loci on DNA.
Fungus	Fungi	An organism like mold, yeast, or mushroom	Farmers treat crops to prevent harmful fungi.
Stylus	Styli	A writing tool or pen-like device	Ancient scribes used metal styli on tablets.
Focus	Foci	The center of interest or activity	The study had two main foci.
Radius	Radii	A line from the center of a circle	The math teacher compared the radii of circles.
Cactus	Cacti	A spiny desert plant	The desert was full of tall cacti.
Alga	Algae	Simple aquatic plants without stems or leaves	Green algae covered the pond’s surface.

Mixed Academic and Scientific Words

Word (Singular)	Plural Form	Meaning	Example Sentence
Antithesis	Antitheses	Direct opposite or contrast	The novel explores several antitheses in human nature.
Axis (geometry)	Axes	A reference line in mathematics or science	Graphs usually show x and y axes.
Nucleus	Nuclei	The central part of an atom or cell	Biologists studied cell nuclei under microscopes.
Papyrus	Papyri	Ancient writing material made from plants	Archaeologists discovered old papyri in Egypt.
Automaton	Automata	A machine designed to act automatically	Engineers built humanoid automata for testing.
Curriculum Vitae	Curricula Vitae	A detailed account of one’s career	Job seekers updated their curricula vitae.
Genus	Genera	A group of related species in biology	Scientists classified plants into different genera.
Larva	Larvae	An immature insect form before adulthood	Butterflies hatch from larvae into adults.
Formula (math)	Formulae	A symbolic rule for calculation	Mathematicians applied several formulae to solve problems.
Stigma (plants)	Stigmata	Markings or symbolic features	Legends told of saints bearing stigmata.

Classical, Academic, and Scientific Plurals

Word (Singular)	Plural Form	Meaning	Example Sentence
Medium (plural sense)	Media	A channel of communication or expression	The news media influenced public opinion.
Stratum	Strata	A layer of rock, soil, or society	Archaeologists studied ancient strata at the site.
Schema	Schemata	An outline or structured framework	Psychologists explained mental schemata in learning.
Bacterium	Bacteria	Microscopic organism found everywhere	Some bacteria are helpful to digestion.
Symposium	Symposia	A formal meeting for discussion	Scholars presented papers at academic symposia.
Thesis (restated)	Theses	A research paper or central argument	The library holds thousands of theses.
Diagnosis	Diagnoses	The act of identifying a disease	Doctors made accurate diagnoses of the illness.
Hypothesis	Hypotheses	A testable idea or explanation	The research tested three hypotheses.
Analysis	Analyses	A detailed examination or study	Multiple analyses confirmed the experiment’s results.
Crisis	Crises	A time of serious difficulty or danger	Economic crises have global consequences.

Advanced Grammar and Science

Word (Singular)	Plural Form	Meaning	Example Sentence
Antenna	Antennae	Sensory organs on insects	Butterflies use their antennae to sense smell.
Antenna (devices)	Antennas	A radio or TV receiver	The building had several antennas on its roof.
Oasis	Oases	A fertile area in a desert	Travelers rested at desert oases.
Phenomenon	Phenomena	An observable occurrence	Scientists study weather phenomena carefully.
Memorandum	Memoranda	A short written message or note	The manager circulated office memoranda.
Index	Indices	A list or reference system	The stock market indices dropped sharply.
Appendix	Appendices	Extra material at the end of a book	The textbook had detailed appendices.
Vertebra	Vertebrae	Small bones forming the spine	Doctors found fractures in two vertebrae.
Papilla	Papillae	Small projections, especially on the tongue	The tongue’s papillae help detect taste.
Addendum	Addenda	An additional note to a document	Contracts sometimes include legal addenda.

Classical and Academic Vocabulary

Word (Singular)	Plural Form	Meaning	Example Sentence
Stimulus	Stimuli	Something that causes a reaction	Bright lights acted as strong stimuli.
Alumnus	Alumni	A male graduate of a school or college	The alumni attended the reunion dinner.
Alumna	Alumnae	A female graduate of a school or college	The university honored its distinguished alumnae.
Radius	Radii	A line from the center to the circle’s edge	The circle’s radii were measured in class.
Formula	Formulae	A rule or equation in math or science	Scientists tested chemical formulae in the lab.
Genus	Genera	A classification of organisms in biology	Several plant genera were found in the forest.
Stratum	Strata	A layer of material or society	Different social strata exist in the city.
Ovum	Ova	A female reproductive cell	The doctor observed multiple ova under the microscope.
Medium	Media	A way or channel of communication	The news media reported the election results.
Corpus	Corpora	A collection of written or spoken texts	Linguists built corpora for language research.

Rare and Technical Terms

Word (Singular)	Plural Form	Meaning	Example Sentence
Crisis	Crises	A time of intense difficulty	The company faced multiple financial crises.
Axis	Axes	An imaginary line around which something rotates	The globe spins on its axes.
Hypothesis	Hypotheses	A proposed explanation for a phenomenon	Scientists tested several hypotheses.
Diagnosis	Diagnoses	The identification of an illness	The doctor made quick diagnoses.
Analysis	Analyses	A detailed examination	The student wrote clear analyses of poems.
Thesis	Theses	A long essay or main argument	The library has many doctoral theses.
Parenthesis	Parentheses	Curved marks used in writing	Insert the dates in parentheses.
Synthesis	Syntheses	The combination of ideas	Researchers created new syntheses of theories.
Basis	Bases	The foundation or starting point	Their bases of argument were weak.
Criterion	Criteria	A standard for judgment	The judges used strict criteria.

Academic and Knowledge Terms

Word (Singular)	Plural Form	Meaning	Example Sentence
Phenomenon	Phenomena	An observable event	Strange phenomena appear in the night sky.
Curriculum	Curricula	A set of courses in study	Schools updated their curricula this year.
Medium	Media	A channel of communication	Social media influence many people.
Memorandum	Memoranda	An official note or message	The office received several memoranda.
Addendum	Addenda	Additional material at the end	The book included two addenda.
Symposium	Symposia	A formal meeting for discussion	Scholars attended three symposia last summer.
Datum	Data	A single piece of information	All the data support the research.
Appendix	Appendices	Material at the end of a book	He checked the appendices for more detail.
Formula	Formulae	A set expression in math or science	Chemists memorized different formulae.
Genus	Genera	A biological classification group	Many genera belong to the rose family.

Science and Research Terms

Word (Singular)	Plural Form	Meaning	Example Sentence
Index	Indices	A numerical measure or list	Stock indices dropped last week.
Vertex	Vertices	A corner point in geometry	The cube has eight vertices.
Matrix	Matrices	A rectangular array of numbers	Engineers solved the matrices quickly.
Radius	Radii	A line from center to circle edge	The radii of both circles are equal.
Nucleus	Nuclei	The central part of an atom	Scientists studied atomic nuclei.
Stimulus	Stimuli	Something that causes a reaction	The experiment used different stimuli.
Fungus	Fungi	A spore-producing organism	The forest was full of fungi.
Cactus	Cacti	A desert plant with spines	The garden displayed tall cacti.
Alumnus	Alumni	A graduate of a school	The alumni gathered for a reunion.
Corpus	Corpora	A collection of written texts	Linguists studied old corpora.

Advanced Terms

Word (Singular)	Plural Form	Meaning	Example Sentence
Oasis	Oases	A fertile spot in a desert	Travelers rested at desert oases.
Larva	Larvae	An immature insect form	The butterfly larvae fed on leaves.
Vertebra	Vertebrae	Bones in the spine	The doctor examined two damaged vertebrae.
Stratum	Strata	Layers of rock or society	Archaeologists dug through ancient strata.
Datum	Data	A piece of information	The scientist checked all data again.
Axis	Axes	A line about which a body rotates	The Earth spins on its axes.
Crisis	Crises	A turning point or emergency	Political crises shook the country.
Thesis	Theses	A long essay or argument	Graduate students submitted their theses.
Analysis	Analyses	A detailed study	His analyses of poetry were brilliant.
Hypothesis	Hypotheses	A scientific guess or explanation	Multiple hypotheses were tested in the lab.

Frequently Asked Questions About Thesis Plural

Writers often ask if a thesis can have different plural forms. The answer is no—the only correct plural is theses. Another question is whether the word can be uncountable, like “research” or “information.” In practice, the thesis is countable, because you can list and number them. You can write “one thesis” or “several theses,” but never “much thesis.”

There is also a question of whether British and American English use the word differently. In this case, both forms of English follow the same rule. Whether you’re writing in the UK, the US, or elsewhere, the plural remains theses. That makes learning easier, since you don’t need to remember regional differences.

Conclusion

The plural of thesis is not regular, but once you know the rule it becomes easy. The singular form is thesis, while the plural form is theses. This change follows a pattern seen in many Greek-based words, where “is” changes to “es.” Understanding this rule helps you avoid mistakes like “thesises” or using “thesis” for both singular and plural.

In academic settings, students usually write one thesis, while professors read many theses. In everyday speech, the word can also mean an idea or argument, and in that case, the plural still stays the same. By practicing with real examples and checking your usage, you can master this word and use it with confidence.