It is synonymous with Gregorian Chant, although its strict interpretation tells us that it is monophonic, meaning it has only one melodic line with no harmony, and is unmetered, meaning it doesn't have regular measured beats to it, while Gregorian Chant is a later development of chant incorporating multiple voices, harmony, and measured notation. Plainsong first developed early in the history of the Christian church around the second and third centuries with the first settings of texts taken from the Psalms.

In the following centuries the number of text settings of this specific chant grew more complex and eventually came to include all the official prayers of the Church, called offices or hours, which are sung daily at the various hours of the day.

For discussion of non Christian chant, investigate Jewish and Hindu musics.
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