A fraternity is a social organization or club, usually for men, whose members consider themselves brothers and often live together. College fraternities are the most well-known type: groups of students who share common interests, support each other academically and socially, and often do community service together. These organizations typically have Greek letters as their names, like Sigma Chi.

Members of a fraternity call each other “brothers” and often form lifelong friendships. They might live in a special house on campus, hold meetings, organize social events, and work together on charitable projects. Joining a fraternity usually involves a selection process and learning about the group's history and values.

The word can also describe any close group that shares a common interest or profession. Teachers might speak of “the fraternity of educators,” meaning the community of people who share that calling. Firefighters, doctors, or craftspeople might describe their professional communities as fraternities, emphasizing the bonds and mutual support among people who do the same work.

Related organizations for women are called sororities, though the word fraternity can sometimes refer to mixed-gender groups as well.