In an antagonistic muscle pair, the one which straightens the limb in question rather than bending it. (Compare flexor muscle.)

An extensor muscle which straightens out a joint. (Compare with a flexor muscle, its opposite)

Any muscle that causes the straightening of a limb is an extensor. This is the opposite of a flexor muscle.

An extensor muscle is any skeletal muscle that opens a joint increasing the angle between components of a limb, such as straightening the knee or elbow and bending the wrist or spine. With the exception of the knee joint the movement is directed backward. This action is known as extension.