Bottom

In a scene the bottom is the one who is on the receiving end of the activities in which they and their partner, the top, are engaged. They may be the one being tied up, the one being flogged, the one having clips or pegs attached to their sensitive bits, etc.

Importantly, the top doesn't ever have carte blanche to do whatever they like with their bottom partner. The bottom retains full authority and must agree in advance to each and every activity before each scene.