The Collaborative International Dictionary of English v.0.48:

Formalin \For"ma*lin\, n. [Formic + aldehyde + -in.] (Chem.)
   An aqueous solution of formaldehyde, used as a preservative
   in museums and as a disinfectant.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

formalin
    n 1: a 10% solution of formaldehyde in water; used as a
         disinfectant or to preserve biological specimens [syn:
         formalin, formol]