Semitone (interval region)
A semitone is half of a tone. It is most commonly used as an unit of interval size with a value equal to 1\12, exactly 100 ¢. An exact semitone in just intonation would be expressed as sqrt(9/8), with a value of about 102 ¢.
Besides the unmodified semitone, used as an interval size measure, we also speak of the diatonic semitone, or minor second, and the chromatic semitone, or augmented unison, of a diatonic scale. These are equated (enharmonically equivalent) if and only if 12edo is used; for example, they are not the same thing in meantone, where the diatonic semitone is larger than the chromatic semitone, or in Pythagorean tuning, where the diatonic semitone is smaller instead.