Half a tone; -- the name commonly applied to the smaller intervals of the diatonic scale.

The smallest interval on the piano, a semitone is the distance between any two consecutive keys. Semitones can be found from a white key to a black key, from a black key to a white key, and in the few cases where there is no black key in between two white keys, from a white key to another white key.

Distance between two adjacent notes on a keyboard, same as half-step.

a unit used in music to describe the ratio in frequency between notes. The unit is actually used in two slightly different ways. In one use, two notes are said to differ by one semitone if the higher note has frequency exactly 16/15 = 1.0667 times the frequency of the lower one. Also, the semitone is used as a synonym for the half step in the standard chromatic scale; in this use, two notes differ by a semitone if the higher note has frequency exactly 21/12 = 1.0595 times the frequency of the lower one.