Brain is the organ inside the head that controls thought, memory, feelings, and activity.
Colloquially, brain can also mean:
1. Intelligence or intellectual ability ("She's got a great brain")
2. The most important person in an organization ("He's the brain behind this project")
3. To hit someone on the head ("I'll brain you if you don't stop")
4. In slang, it can refer to ejaculation ("He lost his brain")