St. Ambros first used it in Milan in 386, and it was adopted soon afterwards in nearly all the Western churches. Another importation from the Eastern to the Western Church This Alleluia, which from the beginning appears to have been a long, melismatic composition, was heard by St. Jerome in Bethlehem, and at his instance was adopted in Rome by Pope Damasus (368-84).
A plainchant is nothing like an opera. Gregorian chant is an important form of plainchant, used mainly in the Roman Catholic Church. With plainchant, all people sing the same music in unison, most of the time. Sometimes, there is a second part, called "organum", which often uses the same melody, but at an interval. This interval is often a fourth or a fifth.