A semitone is the smallest step you usually use in Western piano music: the pitch gap between two keys that sit next to each other, black or white. From C to C-sharp is a semitone; from E to F — no black key in between — is also a semitone. Twelve semitones make an octave. British teachers say semitone; Americans often say half step or half-tone. A whole tone (sometimes just called a tone in this music sense) is two semitones. Though actually, singers and violin players can slide through pitches smaller than a semitone, but the piano, and most pop harmony, is built on this half-step grid. The word is at home in lessons, exams, and choir practice, not in ordinary chat about “tone of voice.”