Frat

A fraternity (from [Latin language|Latin]] : "brother"; ""), fraternal order or fraternal organization is an organization, a society or a club of men associated together for various religious or secular aims. Fraternity in the Western concept developed in the Christian context, notably with the religious orders in the Catholic Church during the Middle Ages. A notion eventually further extended with the middle age guilds, followed by the early modern formation of gentlemen's clubs, freemasons, odd fellows, student fraternities, and fraternal service organizations. Members are occasionally referred to as a brother or – usually in religious context – Frater or Friar.

Today, connotations of fraternities vary according to context including companionships and brotherhoods dedicated to the religious, intellectual, academic, physical or social pursuits of its members. Additionally, in modern times, it sometimes connotes a secret society especially regarding freemasonry, odd fellows, various academic and student societies.

Although membership in fraternities was and mostly still is limited to men, ever since the development of orders of Catholic sisters and nuns in the Middle Ages and henceforth, this is not always the case. There are mixed male and female fraternities and fraternal orders, as well as wholly female religious orders and societies, or sororities. Notable modern fraternities or fraternal orders that with time have evolved to more or less permit female members, include some grand lodges operating among freemasons and odd fellows.

...
Wikipedia