Semitone
Musical intervals are defined by a ratio of frequencies. An octave is a ratio of 2:1, so from 100 Hz to 200 Hz, and from 200 Hz to 400 Hz, are both octaves. A semitone is a ratio of the 12th root of 2 to 1, which is equal to 1.05946:1, so 1000 Hz and 1059.46 Hz are a semitone apart. A musical interval sounds better if the two notes have a small integer ratio. For this reason, it can be approximated by a ratio of 16:15 in just intonation.