The Collaborative International Dictionary of English v.0.48:

Silica \Sil"i*ca\, n. [NL., from L. silex, silics, a flint.]
   (Chem.)
   Silicon dioxide, SiO?. It constitutes ordinary quartz (also
   opal and tridymite), and is artifically prepared as a very
   fine, white, tasteless, inodorous powder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

silica
    n 1: a white or colorless vitreous insoluble solid (SiO2);
         various forms occur widely in the earth's crust as quartz
         or cristobalite or tridymite or lechatelierite [syn:
         silica, silicon oxide, silicon dioxide]