As well as a horizontal axis rotation, usually of a human or other animal, and a changing of digits used in a temporal measurement such as the Y2K rollover, or another kind of systemic change, rollover is also used to describe the effect of an element in a multimedia system such as a web page, which changes in response to cursor movement (a.k.a. mouseover), not requiring a click, or mousedown event. Clients love rollovers. I once had a clueless director who kept pointing out web pages that had a certain dynamic je ne sais quoi, which he couldn't define, and pointing out other pages that lacked this indefinable design flavour. Turned out that he meant rollovers.