Criterion – Criteria

The word "criterion" refers to a standard or principle by which something may be judged. Its plural form is "criteria."

Datum – Data

"Datum" is a single piece of information, whereas "data" is the plural form, referring to multiple pieces of information.

Cactus – Cacti

A single desert plant is a "cactus," but when there are several, they are called "cacti."

Thesis – Theses

"Thesis" is a statement or theory, with "theses" as its plural.

Appendix – Appendices

An "appendix" is a section added to the end of a book. Its plural, "appendices," refers to multiple such sections.

Alumnus – Alumni

An "alumnus" is a male graduate, with "alumni" being the plural form for a group of male graduates or a mixed-gender group.

Analysis – Analyses

"Analysis" is the process of examining something in detail. "Analyses" is the plural.

Medium – Media

"Medium" as a singular noun refers to a means of communication, while "media" is the plural form.

Bacterium – Bacteria

A single microorganism is a "bacterium," with "bacteria" being the plural.

Ox – Oxen

An "ox" is a type of cattle, and "oxen" are multiple such animals.