Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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