The term alma mater means “mother of Alma.”
Nobody knows why there is a generic term for mother of Alma (alma mater) and not, say, mother of Tifany (tifany mater), mother of Poindexter (poindexter mater) or mother of Brandi (brandi mater), but so be it. Currently, the speculation centers around the conjecture that a mother of a particular Alma offered to sleep with a lexicographer on the condition that he honoured her and all mothers of all Almas with a generic term in the dictionary. We don’t know if that’s the real explanation, but it sounds plausible.
One hopes that every Alma is the only child in his or her (the name is not gender-specific) family. Otherwise, the sibling(s) of Alma will undoubtedly be saddled with inferiority complexes and fears that their mother doesn’t love them as much as she loves Alma.