Fraternity refers to:
* A social organization for male students, typically at a college or university. These organizations are often associated with specific rituals, traditions, and social activities, and they can play a significant role in the social life of a campus.
* A brotherhood or close association of people. This broader meaning emphasizes the sense of shared values, goals, or interests.
Therefore, when someone says "frat," they are usually referring to a fraternity or a member of a fraternity.