Source : Webster's Revised Unabridged Dictionary (1913)

Diesis \Di"e*sis\, n.; pl. {Dieses}. [NL., fr. Gr. ?, fr. ? to
   let go through, dissolve; dia` through + ? to let go, send.]
   1. (Mus.) A small interval, less than any in actual practice,
      but used in the mathematical calculation of intervals.

   2. (Print.) The mark [dag]; -- called also {double dagger}.