n. In musical acoustics: Same as combinational tone (which see, under tone), One of a series of tones reckoned downward from a given tone in exactly the same way that the series of overtones or harmonics is reckoned upward. The interval from the given tone to the first undertone is an octave; to the second, an octave and a fifth;n. A low or subdued tone; a tone less forcible than is usual, as in speaking: as, to say something in an undertone.n. A state or degree of tone, as of the physical or mental faculties, below their usual condition. The color of a pigment when seen in very thin layers on a white or light-colored surface.