What is the movement called when the thumb crosses the palm to touch the fingertips?

The movement when the thumb crosses the palm to touch the fingertips is known as opposition. This action is unique to the human thumb, allowing for a high degree of dexterity and the ability to grasp objects effectively. In opposition, the thumb rotates and moves toward the other fingers, enabling functions such as pinching and gripping.

This movement is essential for various tasks, such as writing, holding utensils, and manipulating small objects. It distinguishes human hand function from that of most other species, highlighting the evolutionary advantage of opposable thumbs in performing complex movements and actions.

Other movements like adduction, flexion, and extension refer to different joint actions and are not specific to the thumb's unique ability to engage with the fingertips in this way. Adduction involves moving a limb closer to the body's midline, while flexion and extension pertain to decreasing and increasing the angle between body parts, respectively. Hence, opposition is the correct term for this specific thumb movement.