Tracing its origin, the term "alumni" comes from the Latin verb "alere" which stands for "bring up" and "nourish". Frequently used in Latin literature, the term "alumni" was common to indicate "people who are nourished by a person who is not a natural parent" such as "foster son" and "apprentice".
Derived from the original meaning of "children nurtured by a person different from the natural parents", the term "alumni" was extended to refer to "people who receive intellectual nourishment at school" and now commonly known as graduates of a school. "Alumnus" is the male singular noun while "alumni" is the male (and commonly general form) plural. To specify one or more female graduates, the respective terms "alumna" (singular) or "alumnae" (plural) are used.
Now you know your 'alumnus' and 'alumna' among your fellow 'alumni'!
Antonio Pace (2003). The Meaning of Alumni. AO Dialogue, Volume 16, Issue II, 9.