Weld joint
A joint that tries to hold two bodies perfectly still relative to each other, as if welded — in practice always slightly springy.
The weld joint locks every relative degree of freedom: no rotation, no translation. Two bodies are meant to behave as one rigid piece. The catch is that it is solved as a soft constraint each time step, so the seam gives a little under load — if you want perfect stiffness it is usually better to merge the bodies into one.
Its real value is that it can break. By letting the joint release when the force exceeds a threshold you build destructible structures: walls that crumble, vehicles that fall apart, bridges that snap. Box2D and most 2D engines provide it; in 3D a fixed joint fills the same role.