noun
cantus firmus; plainsong; plainchant
Music term for a pre-existing melody used as the basis of a polyphonic composition, especially in medieval and Renaissance music.
このミサ曲はグレゴリオ聖歌の定旋律に基づいている。
This mass is based on a Gregorian chant cantus firmus.
Compound of 定 (fixed) and 旋律 (melody), likely a calque from Western music terminology.