link (rigid body)

A link is one solid, unbending piece of a robot's body — the stiff bone between two joints. Hold up your own arm: your upper arm is one link, your forearm is another, and your hand is a third. In a robot, a link is usually a bar, plate, or shaped piece of metal or plastic that keeps its exact shape no matter how hard the robot pushes or carries. We call it a rigid body because we treat it as something that does not bend, stretch, or squash; every point on it stays the same distance from every other point.

Links are the structural building blocks of a robot arm or leg. They do not move on their own; they simply hold things at fixed distances and angles so that the joints connecting them can do the moving. Because a link's length and shape are fixed and known, engineers can do clean geometry: if you know which way each joint is turned and how long each link is, you can work out exactly where the robot's hand ends up in space. That tidy, predictable geometry is the whole reason we pretend links are perfectly rigid, even though real metal flexes a tiny bit under load.