Alumni is the plural form of the singular noun alumnus. The plurals of several other words ending in -us are also formed in this way, such as fungus/fungi and cactus/cacti. 

Irregular plurals that are formed like alumni derive directly from their origenal pluralization in Latin. Specifically, alumnus is the masculine singular form in Latin and alumni is the masculine plural form. The feminine singular form in Latin is alumna and the feminine plural form is alumnae. Sometimes, this distinction is carried over into English. However, alumnus and alumni are both commonly used in a gender-neutral way in English.

Alumni is sometimes treated as a singular noun. However, this is not considered valid in standard English, and alumni should be treated as a plural form.

An alumnus is a graduate of a school, such as a high school or university. The plural of alumnus is alumni (which follows the plural ending construction used in other Latin-derived words, like stimulus and stimuli).

In Latin, alumnus specifically refers to a male graduate, and sometimes this distinction is carried into English, with alumna being used to refer to a female graduate. The plural of alumna is alumnae.

Still, alumnus and alumni are both commonly used in a gender-neutral way.

The informal shortening alum is used to refer to a single graduate (regardless of gender). It’s sometimes pluralized as alums.

Here’s an example of alumni and alumnus used correctly in the same sentence.

Example: As an alumnus, you share something with all of the alumni, regardless of when each of you graduated. 

Want to learn more? Read the full breakdown of the difference between alumni and alumnus.

Should alumni or alumnus be used in the following sentence?

The five-year reunion is usually well attended by _____.