The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Print.) The mark [dag]; -- called also double dagger.
      It is used in printing to indicate a cross reference or
      footnote

   Syn: double obelisk.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diesis
    n 1: a character used in printing to indicate a cross reference
         or footnote [syn: double dagger, double obelisk,
         diesis]