Kinktionary
More like guidelines and a lot less like definitions.
Dominant
In contemporary kink a Dominant is a person who consensually receives power, authority, or control in a relationship, scene, or activity.
Core BDSM
In core (or traditional) BDSM the role of a Dominant is more specific. The difference between Dominant and submissive (D/s) and Master and slave (M/s) dynamics is the extent of the Power Exchange.
In a core BDSM D/s relationship consent is considered to be irrevocable as it is given to the negotiated dynamic rather than to individual actions. If consent is revoked or changed then the dynamic is considered broken - similar to a marriage ending in divorce. A Dom who violates a hard limit may be denied the privilege of remaining in their community due to them not upholding the values and traditions demanded of them.
Abbreviations
Dom can be used of someone of any gender but is most commonly used by male-identifying gender. Domme is most commonly used by female identifying Dominants.