A fraternity is a group of people who work in the same industry or have similar careers. Your dad's fraternity of firefighters holds lots of charity events and get-togethers.
Most often, a fraternity is a social club or group made up of men, but it can refer to any organization of people with something in common. At a college or university, a fraternity is a club for men that's by invitation only. Members of these frats, as they are called, often live together in one house and socialize primarily with each other. Fraternity comes from the Latin word fraternitatem, meaning "brotherhood."