Someone involved in the BDSM Lifestyle, that takes on both roles, top or bottom.
A person who can participate in various 'role playing' as either the top or bottom. Someone who may have no clear preference, though purists will argue they aren't true BDSM players.
A 'switch' can be simply two like minded persons, giving each other the opportunity to experience their fantasy.
In some instances, a switch can be a committed 'top' with one partner, but with another is a confirmed 'bottom'.
Depending on mood, or inclination, a 'switch' will be the aggressor, or the submissive. In some instances it is also a form of giving the other committed partner an opportunity to experience their own fantasy.