Someone who takes both sides of a BDSM dynamic — dominant and submissive, top and bottom — moving between them by partner, scene, or mood.
A switch is not someone who couldn't make up their mind. Switching is its own thing: the capacity — and the appetite — to take either side of a scene. BDSM actually runs on two separate axes: power (dominant/submissive, who's running the show) and action (top/bottom, who does and who receives). A switch can slide along either one, or both — which is exactly why "top" and "Dom" aren't synonyms, and neither are "bottom" and "sub." Service tops and power bottoms exist for a reason.
Flexibility comes in grades. Studies of kink populations put switches somewhere between a sixth and a third of the scene, which is a lot for a role people still treat as exotic. But being a switch doesn't mean a 50/50 split on tap: plenty lean heavily one way and only visit the other, some can't flip with the same partner, and some need time between roles rather than a hard cut. Then there's the party trick the textbooks mostly missed — two switches swapping roles mid-scene, handing the reins back and forth in real time.
One distinction worth keeping: switching is usually about power (Dom one night, sub another), while "versatile" — "vers" in queer shorthand — is about the mechanics, who's on which end. People blur the two constantly; you don't have to. Worth knowing too that older, more hierarchical corners of the scene didn't really recognise switching as an identity at all: if authority was something you earned and became, you couldn't hand it back on Tuesday. The modern reading — roles as preferences you own, not ranks you climb — is what gave the switch room to exist.