Cantus Firmus (ディフェンスソング, Difensu Songu?, lit. Defense Song) is Songstress's Sing ability in Final Fantasy X-2. It bestows Defense +1 on each use for 4 MP, and multiple uses stack up to Defense +10. It requires Battle Cry to be unlocked and 10 AP to master.
Cantus Firmus is a phrase of Latin origin and means "fixed melody". It is a pre-existing melody that is used as the basis for polyphonic music and the starting point for species counterpoint.