A rib of celery is a single piece pulled off the larger head or bunch of celery. Another term for a rib is a leafstalk. A midrib of celery is the portion of the rib between the root and the node from which the leafy part sprouts.
Although rib and leafstalk are the proper terms for a single piece of celery, many Americans use the term stalk to refer to one piece. Botanically, stalk refers to the entire head. Because of the confusion around the term stalk, commonsense is necessary to interpret recipes. If the recipe is for a large quantity or a celery-specific dish, stalk most likely refers to a whole bunch of celery. Otherwise, it refers to a single piece.