monotonic (adj.)

in music, etc., "of or pertaining to a single, unvarying note," 1797; see mono- + tonic (adj.). Related: Monotonically.

The secondary sense of monotonous (same or tedious) has so nearly swallowed up its primary (of one pitch or tone) that it is well worth while to remember the existence of monotonic, which has the primary sense only. [Fowler, 1926]