Spring joint
A soft constraint that links two bodies with a spring: it pulls them toward a rest state rather than holding them rigidly.
The force follows Hooke's law — proportional to how far the body has been pulled from its rest position — almost always with a damping term that would otherwise leave the system bouncing forever. Unlike a rigid joint the distance is allowed to vary, and the stiffness is set by the spring constant.
Typical uses are vehicle suspension, bouncy cameras, bungees and cloth-like chains of bodies. Unity calls it a SpringJoint; Bullet offers the equivalent in its six-degree-of-freedom spring joint. Soft constraints are forgiving to solve but can turn unstable at high stiffness and long time steps.