Definitions of brain:

• noun:   that part of the central nervous system that includes all the higher nervous centers; enclosed within the skull; continuous with the spinal cord
• noun:   mental ability

  Example: "He's got plenty of brains but no common sense"

• noun:   the brain of certain animals used as meat
• noun:   that which is responsible for one's thoughts and feelings; the seat of the faculty of reason
• noun:   someone who has exceptional intellectual ability and originality
• verb:   kill by smashing someone's skull
• verb:   hit on the head
• name:  A male given name (rare: 1 in 7692 males; popularity rank in the U.S.: #602)
• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #18792)