(of a cadence) progressing from the subdominant to the tonic chord, as in the Amen of a hymn
2.
(of a mode) commencing upon the dominant of an authentic mode, but sharing the same final as the authentic mode. Plagal modes are designated by the prefix Hypo- before the name of their authentic counterparts: the Hypodorian mode
[C16: from Medieval Latin plagālis, from plaga, perhaps from Greek plagos side]