Source : Webster's Revised Unabridged Dictionary (1913)

Smelling salts \Smell"ing salts\
   An aromatic preparation of carbonate of ammonia and, often,
   some scent, to avoid or relieve faintness, headache, or the
   like.

Source : WordNet®

smelling salts
     n : a pungent preparation of ammonium carbonate and perfume;
         sniffed as a stimulant to relieve faintness