The actual definition of plainsong
Plainsong is usually synonymous with Gregorian chant and is essentially the unharmonized chant of the medieval Christian liturgies in Europe and the Middle East.
There have been four main dialects of plainsong that have been developed - Ambrosian, Roman, Mozarabic, and Gallican. Each dialect seems to have been derived from similar sources.
During the late Middle Ages and the Renaissance, and also in later times, the chant melodies were used as the basis for polyphonic composition. The Benedictine monks of Solesmes sought to restore the Gregorian chant to its original form and their published editions from 1889 onward ended up as the official music of the Catholic Church. The texts of plainsong are the words of the Mass, the Psalms, canticles, and certain verse hymns.
The memory of Plainsong by The Cure
I remember that during my university years, I hardly had a dime to spend on basic necessities, much less spend too much on music. Lucky for me, however, that the University of Ottawa had a Music Listening Room which was basically a lounge with a central area and four semi-private rooms. Each room had its own sound system that was controlled by the attendant in the central area. The vast collection of music that they had came from donations and also from the campus radio station (CHUO).
One day, I was sitting in a semi-private room listening to some Nirvana when the album ended and another album was put on - "Disintegration" by The Cure. From that moment on, I became a Cure fan. I never really listened to any of it, thinking that it was the stuff that my crazy uncle listened to most of the time so it couldn't be good. However, when "Plainsong" came on, I was in awe. Simply put, it was the most hauntingly beautiful song that I had ever heard then and have ever heard since, and the first song to bring tears to my eyes.
Those moments are extremely rare...when a song just totally enraptures your senses that you cannot do anything but sit there stunned. To this day, I have yet to experience that feeling again over any other composition.