mistress
n. countablen. a woman who has a long-term sexual relationship with a person who is already married to someone else.
n. a woman involved in a long-term extramarital relationship with a married partner. Historically used as a title of respect for a woman in authority, though this sense is now largely archaic.
The king's mistress lived in a private house near the palace.
The politician's career ended abruptly when the press discovered he had been supporting a mistress for several years.
In many classic novels, the figure of the mistress serves as a complex foil to the wife, representing a life of passion that exists outside the rigid boundaries of social law.
In modern usage, the term almost exclusively refers to the partner in an affair; the historical sense of 'female head of household' is now rare.