The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ad valorem
    adv 1: in proportion to the estimated value of the goods taxed;
           "the goods were taxed ad valorem" [syn: ad val, ad
           valorem]