canto fermo

cantus firmus (ˈkæntəs ˈfɜːməs)

/ (ˈkæntəʊ ˈfɜːməʊ) /

noun

a melody that is the basis to which other parts are added in polyphonic music
the traditional plainchant as prescribed by use and regulation in the Christian Church

Word Origin for canto fermo

Italian, from Medieval Latin, literally: fixed song