(1) Refers to a signal which has a particular and usually steady pitch. The simplest of tones are sine waves.
(2) Tone also refers to timbre, or quality of a musical sound, such as “bright,” “mellow,” etc.
(3) A synonym for whole-step, an interval of two half-steps, e.g., from C to D. (4) A synonym for note, i.e., any particular sound.