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

PRESERVATIVE is a 12 letter word that starts with P.

1. \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.
   
   

2. \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.