A switch is a person who enjoys acting both the giving and receiving roles of a BDSM or spanking scene — a person who functions as both a top and a bottom, or as both a spanker and a spankee, though of course not at the same time.
The act of changing the role from top to bottom, or vice versa, is called to switch (roles).
In most cases, a switch will stick to a single role within a given scene, although another scene with roles reversed, or with a different partner and the switch in the opposite role, may follow shortly afterwards. In some cases, a switch will stick to a given role with a particular partner, and only switch roles by changing partners. In other cases, two partners may exchange roles from time to time.