What is the meaning of Plainsong?

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

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

The simple notes of an air, without ornament or variation.

A plain unvarnished statement, without exaggeration.

Source: wiktionary.org