mistress

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A woman who has a continuing sexual relationship with a man who is married to someone else.
• noun A woman in a position of authority, control, or ownership, as the head of a household.
• noun A woman who owns or keeps an animal.
• noun A woman who owns a slave.
• noun A woman with ultimate control over something.
• noun A nation or country that has supremacy over others.
• noun Something personified as female that directs or reigns.
• noun A woman who has mastered a skill or branch of learning.
• noun Used formerly as a courtesy title when speaking to or of a woman.
• noun Chiefly British A woman schoolteacher.

from The Century Dictionary.

• noun In mining, a cover for sinkers in a wet shaft; a cover for a sinker's lamp.
• To attend as a lover upon a mistress; pay court to women.
• To become mistress of.
• noun A woman who has authority or power of control, as over a house or over other persons; a female head, chief, or director; a woman who is served by or has the ordering of others: the feminine correlative of master: as, the mistress of a family or of a school. It is also extended to things which are spoken of as feminine.
• noun A title of address or term of courtesy nearly equivalent to madam, formerly applied to any woman or girl, but now chiefly and specifically to married women, written in the abbreviated form Mrs. (now pronounced mis′ ez), and used before personal names. In English lawit is the proper style of the wife of an esquire or gentleman. See miss.
• noun A woman who has mastered any art or branch of study: used also of things.
• noun A woman who is beloved and courted; a woman who has command over a lover's heart; a sweetheart: now used only in poetic language or as an archaisrn.
• noun A woman who illicitly occupies the place of a wife.
• noun In the game of bowls, the small ball at which the players aim; the jack.

from the GNU version of the Collaborative International Dictionary of English.

• intransitive verb obsolete To wait upon a mistress; to be courting.
• noun A woman having power, authority, or ownership; a woman who exercises authority, is chief, etc.; the female head of a family, a school, etc.
• noun A woman well skilled in anything, or having the mastery over it.
• noun Poetic A woman regarded with love and devotion; she who has command over one's heart; a beloved object; a sweetheart.
• noun A woman filling the place, but without the rights, of a wife; a woman having an ongoing usually exclusive sexual relationship with a man, who may provide her with financial support in return; a concubine; a loose woman with whom one consorts habitually.
• noun A title of courtesy formerly prefixed to the name of a woman, married or unmarried, but now superseded by the contracted forms, Mrs., for a married, and Miss, for an unmarried, woman.
• noun Scot. A married woman; a wife.
• noun The old name of the jack at bowls.
• noun to be exempt from control by another person.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A woman, specifically one with great control, authority or ownership.
• noun A female teacher.
• noun A female partner in an extramarital relationship, generally including sexual relations.
• noun A dominatrix.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a woman master who directs the work of others
• noun a woman schoolteacher (especially one regarded as strict)
• noun an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man