Final answer:
The medieval music consisting of Gregorian chant and an additional melodic line to create harmony is known as Organum, an early form of polyphony.
Step-by-step explanation:
The term applied to medieval music that consists of Gregorian chant and an additional melodic line to create harmony is Organum. Organum was an early form of polyphony and one of the first types of music to use measured rhythm. Initially, it began as the addition of a single melodic line or voice, parallel to the chant at a consonant interval, later developing into more complex forms including independently moving lines.
Organum evolved during the medieval era, particularly in the high medieval era, and became more prevalent by the late thirteenth and early fourteenth centuries. It served as a significant musical practice that foreshadowed the styles and compositional techniques that characterized the later Renaissance music, which embraced polyphony and a smoother harmonic sound involving the use of thirds.