What type of motion refers to the straightening of a joint, as opposed to bending?

The term that refers to the straightening of a joint is 'extension.' When a joint extends, it moves in a way that increases the angle between the bones involved in the joint movement, effectively straightening the limb or body part. This action happens in various joints, such as the elbow and knee, allowing for the limb to return to a neutral or straightened position from a bent position.

In contrast, flexion is the opposite movement, which involves bending the joint and decreasing the angle between the bones. Adduction and abduction are terms used to describe movements that involve the limbs moving toward or away from the midline of the body, respectively, but do not pertain to the straightening action of a joint. Therefore, extension is the correct term for the straightening motion.