Kinematic Link or an Element:

Machines contains several material bodies, each one of them being said link or kinematic link or an element. This is a resistant body or an assembly of resistant bodies. The deformation, if any, because of application of forces is negligible. If a link is built of several resistant bodies, they might form one unit with no relative motion of parts with respect to each other.

For instance, piston, piston rod and cross head in steam engine contain different parts however after joining together they do not contain relative motion w. r. t. each other and they form one link. Likewise, belts, ropes, fluid in hydraulic press, etc. undergo small amount of deformation that, if neglected, shall work as resistant bodies and, so, can be called links.