What does the term "extension" refer to in anatomy?

The term "extension" in anatomy specifically refers to the movement of a joint that increases the angle between two bones. This can often be visualized in actions such as straightening the arm at the elbow or straightening the leg at the knee. When a joint is extended, the distance between the two bones connected at that joint increases, distinguishing it clearly from other movements like flexion, which would reduce that angle.

In the context of the other choices, each represents different types of movements. Decreasing the angle (the first choice) describes flexion instead of extension. Rotating a joint around an axis (the third choice) refers to a different motion entirely, known as rotation. Lastly, bringing two body parts closer together (the fourth choice) is indicative of adduction, which is the opposite of extension. Thus, the term aligns correctly with the concept of increasing the angle between the bones at a joint.