Lisa del Giocondo
The title "Mona Lisa" came about in 1550 when Giorgio Vasari, who knew Francesco's family, mentioned that Leonardo painted it for Francesco del Giocondo, Lisa's husband. In Italian, it's called "La Gioconda," the feminine form of her married name. In French, it's known as "La Joconde," which also carries the meaning of "the happy one", derived from the word "jocund".