Final answer:
A musical interval smaller than a semitone or half step is called a microtone, often found in non-Western music traditions.
Step-by-step explanation:
The musical interval smaller than a semitone, or half step, is known as a microtone. Microtones are intervals that are less than a half step and are used in various musical traditions around the world to create a rich tapestry of sound not commonly found in Western music, which typically uses the 12-tone equal temperament system where the octave is divided into 12 equal semitones. Music that incorporates microtones can often sound unusual to ears that are not accustomed to it because it explores pitches that lie 'in between' the notes of the piano.