Dominant is a BDSM term for someone who exercises consensual control (called dominance) over a submissive.
Female dominants are sometimes called Dommes or dommes and male dominants are sometimes called Doms or doms. Female dominants are also sometimes called dominatrices (for just one, dominatrix) although that is usually reserved for professional female dominants (also called pro-dommes, which in the masculine is pro-doms.) Acts of female dominance are often called femdom, and acts of male dominance are often called maledom.
The main difference between a dominant and a top is that the dominant does not follow instructions, although they are limited by what the submissive is willing to do, and does not necessarily give rather than receive acts. The top may sometimes be the partner who is following instructions, i.e., they top when, and in the manner, requested by the bottom. Contrast this with the pure dominant, who might give orders to a submissive, or otherwise employ physical or psychological techniques of control, and might instruct the submissive to perform the act on them.
A Mistress or Master is a dominant in a relationship with a submissive where the submissive consensually gives up total control of one or more aspects of their life to the dominant. Such a submissive is often called a slave. The slave will often accept a collar from their Mistress or Master to show that they are owned.
Some common acts of dominance for a dominant to do to a consenting submissive include flogging, spanking, verbal insults, and bondage.
Acts of female dominance are often called femdom, and acts of male dominance are often called maledom.