A graduate of any gender.

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Alumnus = usually single male graduate, less often single female graduate. A woman graduate or former student is referred to as an alumna.

Macomb, IL 61455 USA.

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Technically, alumniis the masculine plural form of the

Traditionally, “alumnus” refers specifically to a singular male graduate and “alumni” is the plural form for a group of male graduates and for a group of male and female graduates.
The phrase "an alumni" is incorrect, just as "an students" would always be.