I will admit that this one has had me struggling for a number of years and when I read an article where one of my well-respected mentors used the word this way, I began to doubt whether I really did understand the meaning.
Let me state here that I am no Latin scholar, and we are dealing with Latin words (or at least words with Latin roots--and the roots ought to be showing by now) so I can still be corrected. However, I did take the trouble to look it up in my daughter's desk dictionary before starting the rant.
When someone graduates from an institution, he (note the gender here) becomes an alumnus. If he happens to be a she, her distinctive becomes alumna.
A group of these distinguished persons becomes alumni (used for all men or a mix of men and women), unless of course the entire group is female. In the case of a female gender-specific group of graduates, we will refer to them as alumnae.
That's simple, isn't it? Well, not so fast. The modern proclivity to abbreviate everything has people referring to me as an alum of Oklahoma Baptist University (I am, by the way, an alumnus of that wonderful institution). But, and I did look it up, alum is an aluminum-based compound often used in medicine to stop bleeding.
The disheartening thing about this is that people who are alumni keep referring to themselves with this incorrect designation.