What does flex mean in medicals terms?

Flexion is the bending of a joint, typically in the limbs, so as to decrease the angle between the bones that make up the joint. It is the opposite of extension, which is the straightening of a joint. Flexion is typically performed by muscles that are located on the front side of the body (anterior side) and extension is typically performed by muscles that are located on the back side of the body (posterior side). For example, when you bend your elbow, you are flexing your arm, and when you straighten your elbow, you are extending it. Flexion is also used to describe the bending of other parts of the body, such as the neck, back, and feet.