Top 50 Obscure Plural Forms in English - Mr. Greg's English Cloud

English is a fascinating language with a rich history, and one of its quirks is the variety of ways it handles pluralization. While we’re all familiar with regular plurals like cats and dogs, there are many obscure and unusual plural forms that often stump even native speakers. Below is a list of 50 obscure or irregular plural words in English, some of which are borrowed from other languages, and others that have simply evolved in unique ways over time.

Table of Contents

1. Alumnus → Alumni

From Latin: Alumnus refers to a male graduate, and alumni refers to a group of male or mixed graduates.

2. Bacterium → Bacteria

Singular: Bacterium; Plural: Bacteria. A common scientific term where the singular is rarely used in casual conversation.

3. Crisis → Crises

Singular: Crisis; Plural: Crises. An irregular plural formed by changing “is” to “es.”

4. Datum → Data

Singular: Datum; Plural: Data. In common usage today, data is often treated as singular, but technically it’s plural.

5. Focus → Foci

Singular: Focus; Plural: Foci. Used in mathematics and science.

6. Cactus → Cacti

Singular: Cactus; Plural: Cacti. Latin-derived plural, though cactuses is also accepted.

7. Octopus → Octopodes

Singular: Octopus; the original plural from Greek is octopodes, but octopi (from Latin rules) and octopuses are also used.

8. Radius → Radii

Singular: Radius; Plural: Radii. Found in geometry and anatomy.

9. Nucleus → Nuclei

Singular: Nucleus; Plural: Nuclei. A common term in both science and mathematics.

10. Appendix → Appendices

Singular: Appendix; Plural: Appendices. Often seen in academic writing.

11. Thesis → Theses

Singular: Thesis; Plural: Theses. Common in academic contexts.

12. Phenomenon → Phenomena

Singular: Phenomenon; Plural: Phenomena. Used in science and philosophy.

13. Stimulus → Stimuli

Singular: Stimulus; Plural: Stimuli. Seen in psychology and biology.

14. Analysis → Analyses

Singular: Analysis; Plural: Analyses. Another example of the “is” to “es” rule.

15. Criterion → Criteria

Singular: Criterion; Plural: Criteria. Often misused as criterions.

16. Antithesis → Antitheses

Singular: Antithesis; Plural: Antitheses. Another “is” to “es” pluralization.

17. Corpus → Corpora

Singular: Corpus; Plural: Corpora. Used in linguistics and biology.

18. Index → Indices

Singular: Index; Plural: Indices. Common in mathematics and academia.

19. Matrix → Matrices

Singular: Matrix; Plural: Matrices. Frequently used in math and computer science.

20. Stratum → Strata

Singular: Stratum; Plural: Strata. Used in geology and sociology.

21. Vortex → Vortices

Singular: Vortex; Plural: Vortices. Found in physics.

22. Axis → Axes

Singular: Axis; Plural: Axes. Common in geometry and physics.

23. Louse → Lice

Singular: Louse; Plural: Lice. An example of a complete internal vowel change.

24. Goose → Geese

Singular: Goose; Plural: Geese. Another internal vowel change.

25. Man → Men

Singular: Man; Plural: Men. A common irregular plural.

26. Woman → Women

Singular: Woman; Plural: Women. Another irregular plural that changes vowels.

27. Mouse → Mice

Singular: Mouse; Plural: Mice. A complete internal vowel shift.

28. Foot → Feet

Singular: Foot; Plural: Feet. A common irregular plural.

29. Tooth → Teeth

Singular: Tooth; Plural: Teeth. Another example of vowel change.

30. Child → Children

Singular: Child; Plural: Children. An irregular plural with a suffix change.

31. Ox → Oxen

Singular: Ox; Plural: Oxen. An old English plural form.

32. Die → Dice

Singular: Die; Plural: Dice. In gambling and gaming, the singular and plural are often confused.

33. Brother → Brethren

Singular: Brother; Plural: Brethren. Used in religious or formal contexts.

34. Cow → Kine

Singular: Cow; Plural: Kine. An archaic plural, rarely used today.

35. Penny → Pence

Singular: Penny; Plural: Pence. Used in British currency.

36. Cherub → Cherubim

Singular: Cherub; Plural: Cherubim. Found in religious texts.

37. Seraph → Seraphim

Singular: Seraph; Plural: Seraphim. Also found in religious contexts.

38. Formula → Formulae

Singular: Formula; Plural: Formulae. Common in math and science.

39. Stadium → Stadia

Singular: Stadium; Plural: Stadia. Used in architectural and sports contexts.

40. Medium → Media

Singular: Medium; Plural: Media. Used in communication and art.

41. Syllabus → Syllabi

Singular: Syllabus; Plural: Syllabi. Common in education.

42. Memorandum → Memoranda

Singular: Memorandum; Plural: Memoranda. Used in business and legal writing.

43. Appendix → Appendices

Singular: Appendix; Plural: Appendices. Common in academic and medical contexts.

44. Automaton → Automata

Singular: Automaton; Plural: Automata. Used in robotics and mechanics.

45. Bacillus → Bacilli

Singular: Bacillus; Plural: Bacilli. Found in microbiology.

46. Genus → Genera

Singular: Genus; Plural: Genera. Used in taxonomy.

47. Curriculum → Curricula

Singular: Curriculum; Plural: Curricula. Common in education.

48. Oasis → Oases

Singular: Oasis; Plural: Oases. Found in geographical contexts.

49. Parenthesis → Parentheses

Singular: Parenthesis; Plural: Parentheses. Used in grammar and writing.

50. Quiz → Quizzes

Singular: Quiz; Plural: Quizzes. An example of doubling the consonant before adding “es.”