modulation (n.)

late 14c., modulacioun, "act of singing or making music, harmony," from Old French modulation "act of making music" (14c.) and directly from Latin modulationem (nominative modulatio) "rhythmical measure, singing and playing, melody," noun of action from past-participle stem of modulari "regulate, measure off properly, measure rhythmically; play, play upon," from modulus "small measure," diminutive of modus "measure, manner" (from PIE root *med- "take appropriate measures"). Meaning "act of regulating according to measure or proportion" is from 1530s. Musical sense of "action or process of changing from one key to another" is by 1690s.