A halfstep, also called a semi-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 halfsteps are equal in size. However, in other tuning systems, they may have different sizes.
In terms of a piano or other keyboard instruments, a halfstep is produced by playing any two adjacent keys, regardless of whether they are black or white. For example, B is adjacent to C and thus the interval between them is a halfstep, as is the interval between C and C sharp.