A motion is a movement or exertion made by a major joint or body link. From the perspective of preventing musculoskeletal disorders, the concern is for the number of loads on, or slides of, a tendon and is the primary criterion for quantifying the motions of a task.
A good rule of thumb is that a motion occurs every time the body part in question changes directions. Thus, the extremely common flexion/extension cycle involves two motions.
From an efficiency standpoint, this approach also works well. It provides a standardized way to compare work methods and provides insights in a way that time study alone does not.
Motions are quantified by observing and counting, either directly or by viewing a video clip. Counting motions can often be a very simple undertaking, since it can be done by the unaided eye and counting to oneself. On the other hand, counting can sometime be quite difficult, since in many tasks it can be troublesome to determine exactly what constitutes a motion.
Getting in the habit of observing motions in this manner helps to identify wasted steps and better vs. worse methods, even if done mentally without tallying the actual counts.
For more details on counting motions, see MacLeod D., 2012, The Rules of Work, Second Edition, pp. 126-129, Taylor & Francis.