A college alumnus is a male who has graduated from a particular college or university. "Alumnus" is the singular form of the word "alumni," which is a plural noun that refers to a group of male graduates or a group of male and female graduates.
When referring to female graduates, one may use the word "alumnae" for a group or "alumna" for one. If a person's gender is not specified, one may also opt to use the words "alum" and "alums." These forms are less formal but do not refer to a specific gender. All forms are Latin in origin.