Alumnus is known as the Latin word used for a singular graduate of an institution usually a male. It might be of a school, university or college.
"My dad is an alumnus of Cambridge University."
Alumni is the plural form of alumnus, obviously meaning a group of male graduates of an institution. Though modern times have changed it to mean graduates of an institution which could be either male or female.
"Peter Parker is the president of the alumni association of his alma mater."