Origins
The word "sorority" entered English in the 16th century from Medieval Latin "sororitās" (sisterhood), from Latin "soror" (sister), which descends from Proto-Indo-European *swésōr (sister). Like "fraternity" (from "frāter," brother), "sorority" elevates a kinship term into a word for organized group membership — but its history is far more recent and more specifically American.
The PIE root *swésōr is remarkably well-preserved across the language family. Latin "soror," Sanskrit "svásar," Old English "sweostor" (modern "sister"), German "Schwester," Old Irish "siur," Russian "sestra," Lithuanian "sesuo" — all descend from the same source. Linguists have proposed that *swésōr may be a compound of *swe- (self, one's own) and a feminine element *-sōr (woman), yielding a literal meaning of "woman of one's own group" or "own-woman." If this is correct, the concept of "sister" originally emphasized belonging — a woman who is one of us — rather than biological sibling relationship.
In Latin, "soror" meant biological sister and, by extension, any close female companion. Roman culture formalized the sibling relationship in legal terms: sisters had specific inheritance rights and obligations. In the Christian tradition, "soror" became the title for nuns — women who were "sisters" in their religious community, just as monks were "brothers" ("fratres," whence "friars").
Development
The modern American usage of "sorority" for women's college organizations developed in the 1880s. The first women's organizations at American universities initially called themselves "fraternities" — a term they had equal right to, since "fraternitas" simply meant "brotherhood" and the concept applied by analogy. Kappa Alpha Theta (founded 1870) and Kappa Kappa Gamma (1870) were called "women's fraternities." The term "sorority" was coined to provide a gender-specific alternative, and it gradually replaced "women's fraternity" in popular usage, though some organizations still officially use the older term.
The Greek-letter naming convention of sororities (Alpha Chi Omega, Delta Delta Delta, Zeta Tau Alpha) follows the pattern established by male fraternities in the late 18th century. The letters are typically abbreviations of Greek mottoes known only to members — a tradition of secret naming that connects these modern organizations to the ancient mystery religions and their initiation rites.
The derivative "sororal" means "of or relating to a sister" — the feminine counterpart of "fraternal." "Sororal polygyny" is an anthropological term for the practice of a man marrying sisters — a marriage pattern found in some cultures where sibling bonds reinforce marital alliances. "Sororicide" — the killing of one's sister — parallels "fratricide" but is far rarer both as a word and as a cultural archetype.