K. asked for an explanation of plainsong in response to one of my recent entries about choir practice. Basically, it's a form of liturgical choral singing, in which the meter is free and there is only one “part” or “voice” (aka unison).
Gregorian Chant is a kind of plainsong, but not all plainsong is chant. Some of it is actually pretty melodic.
Here's a more technical description, from Wikipedia.org:
Broadly speaking, plainsong is the name given to the body of traditional songs used in the liturgies of the Catholic Church. The liturgies of the Orthodox Church, though in many ways similar, are generally not classified as plainsong, though the musical form is nearly as old as Christendom itself.
Plainsong is monophonic, and is in free rather than measured rhythm. Gregorian chant is a variety of plainsong that was standardized by Pope Gregory I in the 6th century CE, and represents the first revival of musical notation after knowledge of the ancient Greek system was lost. Plainsong notation differs from the modern system in having only four lines to the staff and a system of note-shapes called neumes.
There was a significant plainsong revival in the 19th century CE when much work was done to restore the correct notation and performance-style of the old plainsong collections, notably by the monks of the Abbaye de Solesmes in Northern France. The use of plainsong is now mostly confined to the Monastic Orders.