The Collaborative International Dictionary of English v.0.48:

Semiditone \Sem`i*di"tone`\, n. [Pref. semi- + ditone: cf. It.
   semiditono. Cf. Hemiditone.] (Gr. Mus.)
   A lesser third, having its terms as 6 to 5; a hemiditone.
   [R.]
   [1913 Webster]