What term describes the straightening of a joint?

The term that describes the straightening of a joint is extension. When a joint is extended, the angle between the bones forming that joint increases, effectively straightening it. This movement is opposite to flexion, where the joint decreases in angle, causing the bones to come closer together. Extension is commonly observed in movements such as straightening the arm at the elbow or the leg at the knee.

To provide further context, flexion involves bending a joint, which is not what is being described in the question. Abduction refers to moving a limb away from the midline of the body, while adduction is the opposite, bringing a limb closer to the midline. Both of these terms pertain to movements in a lateral direction and do not relate to the action of straightening a joint. Thus, extension accurately defines the straightening action at a joint.