Roles in a consensual power dynamic — one directs, the other yields — for a scene or as an ongoing relationship.
D/s is the "power" half of BDSM — the part that's less about rope or impact and more about who's steering. The Dominant leads; the submissive follows. What makes it kink rather than coercion is that the submissive is the one who hands the keys over, and can ask for them back. The written form carries this in its typography: capital D, lowercase s, slash in the middle. Big letter for the one with authority, small letter for the one who yields — a convention that spread through BDSM chatrooms so you could read someone's role at a glance.
The common misconception is that the Dominant holds all the power. In practice the community tends to say the opposite: the submissive sets the limits, names the safeword, and defines the whole scope of what's on offer. Control is lent, not surrendered. A scene runs on negotiation, a safeword, and aftercare — remove those and you don't have D/s, you have a problem.
D/s also isn't one fixed thing. It can live entirely inside the bedroom (erotic power exchange) or extend across a whole relationship (total power exchange, TPE — a term traced to the 1990s Usenet group alt.sex.bondage). Titles vary too: Dom/sub, Master/slave, Daddy/Dom and more. And the roles aren't a personality verdict — plenty of people who run boardrooms by day submit by night, and vice versa. The label describes an agreement, not a character.