"A copula is a rapid, connected discant..."[1] However, it is often considered to be a particular type of polyphonic texture similar to organum, but with modal rhythm.
This style is typical of what is referred to as Notre Dame Polyphony; examples of which can be found in the Magnus Liber Organi.
Copula might have implied a strophic construction with much repetition in the various parts, which was characteristic of much of the music written in this idiom.
It is, in essence, the "coming together" of these two (or more) parts at the cadence that led to the term copula being used, from the Latin meaning "that binds."
He further describes it as a fast, cadential passage that is similar to either the 2nd or 6th rhythmic mode, although it differs in tempo and notation.