Adjective
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.
Source: Webster's dictionaryInstead, it would be based mostly on a plagal cadence —an IV-I change (in C: F-C). Source: Internet
In the plagal modes, its position is somewhat irregular. Source: Internet
The reciting tones of all authentic modes began a fifth above the final, with those of the plagal modes a third above. Source: Internet