Definition of "plainsong"

(music) A form of monophonic chant in unison using the Gregorian scale, sung in various Christian churches.  examples

(music) A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song.  examples

(music) The simple notes of an air, without ornament or variation.  examples

(by extension) A plain unvarnished statement, without exaggeration.  examples