maîtresse en titre
/meɪˌtrɛs ɒn ˈtiːtrə/ /mɛˌtrɛs ɒn ˈtiːtrə/(US maîtresse en titer)
noun
An official or acknowledged mistress.
Origin
Mid 19th century; earliest use found in William Thackeray (1811–1863), novelist. Apparently from French maîtresse en titre (although this expression not recorded in French dictionaries) from maîtresse + en titre.