To shape one signal by another — a voice through knobs, a wave riding a wave. The turning of a sound into its other selves without losing the throat it came from.
Latin modulari (to measure out rhythmically, to make melody, to regulate), from modulus (a small measure); to shape one signal by another without losing the source it came from.
One signal shaped by another until it becomes something else, while still carrying the source it never actually left.