1. A British person or somebody born and raised in Great Britain.
2. Some times used either as a friendly term or insulting term to describe a person of British descent, depending on how the particular person precieves it.
American: Look at that guy, he's awesome!
American #2: Oh, yeah. He's a Brit, isn't he?
American: Yup. That is what makes him more awesome; he's a Brit!
American #2: Isn't the term "Brit" offensive?
American: Not that I know of; a Brit is someone described as an individual of British ancestory, no more, no less.