The person who appears to be in charge, during sexual role playing, usually in a BDSM setting. Does not always imply the person on top, during intercourse.
Dom, Dominant, are other words associated with this position of authority in the BDSM lifestyle.
Dominatrix is generally in reference to a female dominant.
While generally associated with the BDSM community, it can simply refer to the more aggressive partner in a sexual relationship.
This being where the wishes of the one, are more accepted, or done versus those of the other.
While it can also signify the one who engages in penetration, it is not necessarily so. It can be the one who insists on being penetrated, dominating the sexual play between the two parties.