The Collaborative International Dictionary of English v.0.48:

Semitransparency \Sem`i*trans*par"en*cy\, n.
   Imperfect or partial transparency.
   [1913 Webster]

WordNet (r) 3.0 (2006):

semitransparency
    n 1: the quality of allowing light to pass diffusely [syn:
         translucence, translucency, semitransparency]