plainsong

or plain song

[ pleyn-sawng, -song ]
/ ˈpleɪnˌsɔŋ, -ˌsɒŋ /

noun

the unisonous vocal music used in the Christian church from the earliest times.
modal liturgical music; Gregorian chant.
a cantus firmus or theme chosen for contrapuntal development.
any simple and unadorned melody or air.

Origin of plainsong

1505–15; translation of Medieval Latin cantus plānus
Also called plainchant (for defs 1, 2).
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for plainsong

plainsong

/ (ˈpleɪnˌsɒŋ) /

noun

the style of unison unaccompanied vocal music used in the medieval Church, esp in Gregorian chantAlso called: plainchant

Word Origin for plainsong

C16: translation of Medieval Latin cantus plānus
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012