The Collaborative International Dictionary of English v.0.48:

Fixative \Fix"a*tive\, n.
   That which serves to set or fix colors or drawings, as a
   mordant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fixative
    n 1: a compound (such as ethanol or formaldehyde) that fixes
         tissues and cells for microscopic study
    2: a varnish dissolved in alcohol and sprayed over pictures to
       prevent smudging