Trope: 1) Interpolations in plainsong words resulting in musical melisma on one note or a fragment of new melody. Practice flourished from 9th to 15th century, was abused, and finally banned by Tridentine reform. Survived only as the sequence (trope set to final melisma of Alleluia).
2) Term used by Hauer to describe 44 pairs of unordered hexachords which are basis of his version of 12-note technique. (Concise Oxford Dictionary of Music.)

I trust that makes it crystal clear.