Trope

A musical addition to or a substitution for an item of plainchant. A trope may or may not have words, and is often an introduction to a chant. an addition to a pre-existent chant (known as the "host"). The trope introduces and comments on the text of the host chant. Tropes are usually syllabic and are sung by a soloist; they may be monophonic or polyphonic. a type of plainchant used between 10th and 12th centuries