Source : Webster's Revised Unabridged Dictionary (1913)

Preservative \Pre*serv"a*tive\, a. [Cf. F. pr['e]servatif.]
   Having the power or quality of preserving; tending to
   preserve, or to keep from injury, decay, etc.

Preservative \Pre*serv"a*tive\, n.
   That which preserves, or has the power of preserving; a
   presevative agent.

         To wear tablets as preservatives against the plague.
                                                  --Bacon.

Source : WordNet®

preservative
     adj : tending or having the power to preserve; "the timbers should
           be treated with a preservative"
     n : a chemical compound that is added to protect against decay
         or decomposition