Definitions for half step
Random House Webster's College Dictionary
Category: Music and Dance
Mil. a step 15 in. (38 cm) long in quick time and 18 in. (46 cm) long in double time.
Origin of half step:
semitone, half step(noun)
the musical interval between adjacent keys on a keyboard instrument
An interval of which there are seven in a fifth or twelve in an octave, consisting of two tones whose frequency ratio is about 1.059 or 18/17.