We see this mistake so often in applicant essays, we thought it warranted explanation. If you get this right, you will appear smart for the rest of your life, so pay attention.
Collectively, you and everyone you graduated with from your institution are alumni. Alumni is the Latin plural form of a word that originally meant foster son or pupil.
Everyone who graduated from your school is a former pupil or “foster son” (or daughter) of that institution. You might even have a sticker on your car that says “State U Alumni,” which is a nod to everyone who graduated as an entire group, male and female alike.
The mistake comes when you try to use that word to refer to yourself as an individual graduate of your school. You are not “an alumni” of State U.
You are either an alumnus or an alumna. While alumni has no gender connotation, the word alumnus is most often used as masculine and alumna is feminine. So each of your fraternity brothers is “an alumnus,” or as the case may be, each of your sorority sisters is now “an alumna.”
There’s nothing that makes someone appear less like a college graduate than to refer to themselves as “an Alumni”—and we all want to appear like college graduates when we apply to business school, right?
A couple of other notes…Long ago, some clever (or lazy) person decided to avoid the risk of looking dumb and chopped off the vowel on the end of the root word, and now refers to himself as “an alum” or collectively as “we alums.” This is technically incorrect, but has been so commonly used that you will indeed find it in the dictionary as the second definition of what “alum” actually is, which is a white powder used in medicine (pronounced differently of course).
"Alum," is often referred to as an "informal" use of the word alumni, which is a nicer way to say it’s wrong.
To further complicate the issue, just in case it ever comes up, if you are trying to refer specifically to a group of women-only graduates, you should use “alumnae,” (pronounced “alum-knee”) which is simply the plural form of “alumna.” Got it? Now you can really consider yourself a college graduate and make us all look good! You’ll also win favor from the admissions committees.