A semitone, also called a half step or half tone, is the interval between any two adjacent notes in the commonly used twelve-tone scale and is the smallest commonly used interval in Western tonal music.
In the now dominant twelve-tone equal temperament tuning system, all semitones are equal in size. However, in other tuning systems, they may have different sizes.
In terms of a piano or other keyboard instrument, a semitone is produced by playing any two adjacent keys, regardless of whether they are black or white. For example, C is adjacent to C sharp and thus the interval between them is a half step.