An alumnus (pl. alumni) according to the American Heritage Dictionary is "a male graduate or former student of a school, college, or university." In addition, an alumna (pl. alumnae) is "a female graduate or former student of a school, college, or university." If a group includes more than one gender, even if there is only one male, the plural form alumni is used.
The English word "alumnus" comes from the Latin noun "alumnus" meaning "nursling" or "ward", in allusion to the metaphorical relationship of the student to Alma mater - the romantic female embodiment of each educational institution. It has come to mean, within common American English usage, a graduate of a seat of learning. "Alumna" is a feminine form of "alumnus" that has entered common English usage.
As indicated in the American Heritage Dictionary, an alumnus or alumna is either someone who has attended the school (or a "former student of a school") or someone who has graduated from the school.  Furthermore, according to dictionary.reference.com and the United States Department of Education, the term alumnae is used in conjunction with either women's colleges or a female group of students. The term alumni is used in conjunction with either men's colleges, a male group of students, or a mixed group of students:
Traditionally, the masculine plural alumni has been used for groups composed of both sexes and is still widely so used: the alumni of The University of Texas. Sometimes, to avoid any suggestion of sexism, both terms are used for mixed groups: "the alumni/alumnae of The University of Texas" or the "alumni and alumnae of The University of Texas" coeducational institutions usually use alumni for graduates of both sexes. Some may prefer the phrase "alumni and alumnae" or the form "alumnae/i", which is the choice of many women's colleges that have begun to admit men.
The term is sometimes shortened to "alum", which stands for "an alumna or alumnus."
"Alumni" (a plural form) is often used as a singular form for both genders; for example, "I am an alumni of the university," as opposed to "I am an alumnus/alumna of the university." This usage is erroneous in formal or historic usage. The prevalence of this usage is likely due to an ignorance of Latin grammar and the fact that printed documents and university merchandise almost always use the plural form of the word.
At most UK independent schools, New Zealand schools, and a few universities in the UK, and to a lesser extent in Australia and Canada, the phrases old boy and old girl are traditionally used for former school pupils, and old member or member (or "alumnus" in New Zealand) for former university students. At the Royal Military College of Canada, the phrases former cadet and member of the old brigade are traditionally used, as are college numbers. Another example is the term old corps, in reference to alumni from the Virginia Military Institute.
Some schools use a specific term clearly linked to the school name, such as "Old Etonian", "Old Knox Grammarian" or "Old Reptonian" (old boys of Eton College, Knox Grammar School and Repton School); or a more obscure one, such as "Old Citizen" and "Old Gregorian" for those of the City of London School and Downside School. Other UK examples include "Old Alleynian" (Dulwich College) and "Old Blue" (Christ's Hospital).
In Scotland the term former pupil (FP) is also used, especially when referring to sports teams of a school. Some US schools prefer former student.