Plagal \Pla"gal\ (pl[=a]"gal), a. [F., from Gr. pla`gios
sidewise, slanting.] (Mus.)
Having a scale running from the dominant to its octave; --
said of certain old church modes or tunes, as opposed to
those called {authentic}, which ran from the tonic to its
octave.
[1913 Webster]
{Plagal cadence}, a cadence in which the final chord on the
tonic is preceded by the chord on the subdominant.
[1913 Webster]