The quadriceps is the big muscle at the front of a person's thigh. Without a quadriceps, you wouldn't be able to stick your leg straight out, walk, jump, or bend.
The word quadriceps is often abbreviated as quad. It works in close conjunction with the knee, helping to extend the leg for moving and walking. This major muscle got the name quadriceps from quadri-, "consisting of four" in Latin, because of its division into four very distinct parts. Quadriceps is literally "four-headed."