The Collaborative International Dictionary of English v.0.48:

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]