Source : Webster's Revised Unabridged Dictionary (1913)

Ad valorem \Ad va*lo"rem\ [L., according to the value.] (Com.)
   A term used to denote a duty or charge laid upon goods, at a
   certain rate per cent upon their value, as stated in their
   invoice, -- in opposition to a specific sum upon a given
   quantity or number; as, an ad valorem duty of twenty per
   cent.

Source : WordNet®

ad valorem
     adv : in proportion to the estimated value of the goods taxed;
           "the goods were taxed ad valorem" [syn: {ad val}]