Hinge joint
A physics constraint that allows rotation about a single axis only, like a door hinge, a knee or a wheel axle.
Of a rigid body's six degrees of freedom this constraint locks five and leaves one rotation. Physics engines such as PhysX, Bullet and Box2D usually call it a revolute joint. It can be fitted with a motor that drives the angle and with limits that stop it from bending the wrong way.
In ragdolls it stands in for joints that only bend one way; in machinery for doors, hatches and crankshafts. The constraint is solved alongside all the others by the engine's constraint solver every time step, most often with iterated impulses.