plainsong

Short definition from Wiktionary

Noun

1. 1
   A form of monophonic chant in unison using the Gregorian scale, sung in various Christian churches.
2. 2
   A cantus firmus or theme chosen for contrapuntal treatment; so called because often an actual fragment of plain-song.
3. 3
   The simple notes of an air, without ornament or variation.
4. 4
   A plain unvarnished statement, without exaggeration.