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late 14c., "act of singing or making music," from Old French modulation "act of making music" (14c.), or 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 (see module). Meaning "act of regulating according to measure or proportion" is from 1530s. Musical sense of "action of process of changing key" is first recorded 1690s.
modulation mod·u·la·tion (mŏj'ə-lā'shən)
The functional and morphological fluctuation of cells in response to changing environmental conditions.
The variation of a property in an electromagnetic wave or signal, such as amplitude, frequency, or phase.