What is the term for the straightening of a joint?

The term for the straightening of a joint is indeed extension. In the context of joint movements, flexion refers to decreasing the angle between two body parts, which often results in bending a joint. Conversely, extension increases the angle between body parts, effectively straightening the joint. For example, when you raise your arm straight out to the side from a bent elbow position, you are extending the elbow joint.

Adduction and abduction describe movements towards and away from the midline of the body, respectively, and are not related to straightening a joint. Therefore, extension is the correct term for the action of straightening a joint.