I agree with Karin that there are multiple definition of exercise depending on who you ask what class you take or what a researcher’s bias is. From a graduate exercise physiology class taken fall quarter 2012:
From the physiological perspective, it is useful to think about what the essential factor is that makes something exercise. I think it is contractions of skeletal muscles. Skeletal muscles exert force to (a) cause movement of the body or a body segment (e.g., lifting a weight, running, swinging a bat), (b) resist movement (e.g., lowering a weight, absorbing a hit in football, catching an object), or (c) prevent movement (e.g., holding the iron cross position on the rings in gymnastics, holding a football lineman’s stance until the ball is snapped). It is the muscle contractions that do the activity we call exercise; without muscle contractions we don’t do any exercise. Muscle contractions are fundamental. Then, a lot of other things have to happen in the body to support the muscular activity: adjustments of the heart’s function, circulation of the blood, lung function, etc.