Final answer:
Flexion is the bending of a joint that decreases the angle between bones, opposite of extension which increases the angle as the joint straightens.
Step-by-step explanation:
Bending parts of a joint so that the angle between parts of the joint is decreased is known as flexion. During flexion, the angle between bones is decreased, which is similar to how bending the elbow reduces the angle between the forearm and the upper arm. Flexion contrasts with extension, where the angle between the parts of a joint is increased as the joint straightens.
For example, flexion includes movement like bringing the hand toward the shoulder, whereas extension involves straightening the arm back to its initial straightened position. An easy way to remember this is that flexion usually involves a 'bending' motion, while extension is associated with 'straightening' movements.