Maг®tresse «Extended»

: It can denote a woman who owns an animal (e.g., la maîtresse d'un chien ) or a female head of a household ( maîtresse de maison ). Usage as an Adjective

: It refers to a woman in a romantic relationship with a man who is often already committed or married. MaГ®tresse

While mostly a noun, maîtresse can function as an meaning "main," "central," or "principal" in specific idiomatic expressions: What does maîtresse mean in French? - WordHippo : It can denote a woman who owns an animal (e