The term alma mater is Latin for “nourishing mother,” and it is used in two different ways in modern English. In the first sense, it is an educational institution, classically a college, and in the second sense, it is an anthem or fight song of an educational institution. Usually in the sense of an educational institution, the term is used in reference to an institution that someone has personally attended. When the term is used to describe an anthem, the speaker need not have attended the institution in question.
Originally, alma mater was used as an honorific for some Latin goddesses who were viewed as kind, nurturing, or loving. In the 1700s, the term came to be adopted in reference to British institutions of higher learning, and the practice spread to other regions of the world. The University of Bologna, the world's oldest continually running institution, even integrated the term into its motto, which is alma mater studorium, or “nurturing mother of studies.”
While this term is most commonly used in reference to colleges, some people use it in discussions of high schools, especially private high schools, preparatory schools, and other institutions that are perceived as more elite than public or ordinary high schools. It is appropriate to refer to any institution of higher learning in this way, whether one has graduated or is still attending.
In the sense of a place of study, this term probably arose as a slangy shorthand. Latin jokes, puns, and epithets were especially popular in the 1700s, and “alma mater” is not the only one to have caught on. Using Latin in the 1700s was an indicator of higher education in Britain, and people threw around terms like this to emphasize their membership in the clubby community of college students, especially in communities where a rivalry between town and gown existed.
The use of this term to describe a fight song or anthem is more recent, although it references the same basic idea of an institution of higher learning as a source of nurturing and knowledge. Many graduates know the alma mater of their colleges, and they may sing along at sports games and other events to express solidarity with the school and its current and past students. Knowing the words can also increase a sense of belonging.