an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
"The husband was furious to discover that his wife had been keeping a mistress in the city for years."
a woman schoolteacher (especially one regarded as strict)
"The headmaster appointed his old friend's daughter as mistress to oversee the rigorous arithmetic lessons."
a woman master who directs the work of others
"In ancient Rome, the mistress of the household managed all domestic affairs and oversaw the servants' daily tasks."
A woman, specifically one with great control, authority or ownership
"After years of being addressed simply as Miss, she finally became a mistress and was introduced to guests as Mrs. Higgins."
Used as the title of a married woman before her name. Now used only in the abbreviated form Mrs.
In plain English: A mistress is a woman who has an ongoing sexual relationship with someone else's husband, boyfriend, or partner.
"The husband was furious when he discovered his wife had hired a mistress to take care of their garden while they were on vacation."
Usage: Use this noun primarily in the historical sense referring to an unmarried female lover rather than for modern contexts involving professional expertise. Avoid confusing it with "mistress" as a verb meaning to master a skill, which is unrelated and rarely used today.
Of a woman: to master; to learn or develop to a high degree of proficiency.
"After years of practice, she became a true mistress of the violin."
In plain English: To be mistress of something means to have complete control over it.
"The rain mistresses the landscape by turning dry fields into muddy puddles."
The word mistress comes from the Old French term for "master," which was adapted into Middle English as a female counterpart to master. Although it can be broken down by modern readers into "mist" and "-ress," its true origin lies in that earlier foreign phrase rather than an English compound of misty weather.