Source : Webster's Revised Unabridged Dictionary (1913)

Plagal \Pla"gal\, a. [F., from Gr. ? 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.

   {Plagal cadence}, a cadence in which the final chord on the
      tonic is preceded by the chord on the subdominant.