Final answer:
Plainchant music is characterized by its monophonic, single-line melody, without instrumental accompaniment, and is a form of medieval church music distinct from later polyphonic Renaissance music.
Step-by-step explanation:
The characteristic that describes plainchant music is that it is characterized by a single-line melody. Plainchant, often referred to as Gregorian chant, is a form of medieval church music that involves monophonic, unaccompanied vocal music. It developed during the early Christian church, with its pure melody lines and free rhythm designed to be sung in a liturgical setting. It is distinct from the polyphonic styles that developed later during the Renaissance period, which involved complex harmonies and the use of multiple, independent vocal lines. Plainchant does not incorporate instruments, unlike many other forms of music from the Renaissance onward. The focus on a modal system, smooth melodic lines, and a single vocal part without accompaniment or harmonization are key elements of plainchant.