The semitone is also the smallest interval between notes in MIDI. And two minor seconds, or two semitones, form a whole tone.
A semitone corresponds to a minor second. On the guitar, semitones are between two frets. On a piano, it corresponds to two adjacent keys - it fits between the white and black keys, or in two cases between two white keys (those that are not separated by a black key). The semitone is the smallest interval between sounds.