alumna / alumnus / alumni / alumnae
The gendered Latin forms for a former student, and the casual short forms.
Alumnus is one man, and alumni is the plural for men or for a mixed group. Alumna is one woman, and alumnae is the plural for women. All four name former students as well as graduates. Do not use alumni for one person. Alum and alums are casualisms, and they do not belong in standard English. For one former student without regard to sex, use graduate.
Examples
Use She is an alumna of Reed.
Use He is an alumnus of Reed.
Use The alumni raised the endowment.
Use Her grandmother and two aunts are alumnae of Reed.
Avoid She is an alumni of Reed.