Kinematic Pair: A pair is a joint of two elements which permits relative motion. The relative motion among the elements of links that built a pair is needed to be fully constrained or successfully constrained.
Kinematic Link or Element: Kinematic link means to a resistant body or an assembly of resistant bodies that go to make a part or parts of a machine connecting other parts that have motion relative to it.
Resistant Body: A body is state to be a resistant body if this is capable of transmitting the needed forces with negligible deformation.
Completely Constrained Motion: While the motion among a pair is limited to a definite direction, irrespective of direction of force applied and just one independent variable is needed to define motion. Such a motion is called to be a completely constrained motion.