hemitonium

hēmĭtŏnĭum, ĭi, n., = ἡμιτόνιον,

• a half-tone, semitone: intervalla tonorum et hemitoniorum, Vitr. 5, 4, 3; Mart. Cap. 9, § 930; 963; Hyg. Astr. 4, 14; Macr. S. 2, 1, 22.