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1530s, from Latin tropus "a figure of speech," from Greek tropos "turn, direction, turn or figure of speech," related to trope "a turning" and trepein "to turn," from PIE root trep- "to turn" (cf. Sanskrit trapate "is ashamed, confused," properly "turns away in shame;" Latin trepit "he turns"). Technically, in rhetoric, a figure of speech which consists in the use of a word or phrase in a sense other than that which is proper to it.
in medieval church music, melody, explicatory text, or both added to a plainchant melody. Tropes are of two general types: those adding a new text to a melisma (section of music having one syllable extended over many notes); and those inserting new music, usually with words, between existing sections of melody and text.