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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AD VALOREM. According to the value. This Latin term is used in commerce in
reference to certain duties, called ad valorem duties, which are levied on
commodities at certain rates per centum on their value. See Duties; Imposts;
Act of Cong. of March 2, 1799, s. 61 of March 1, 1823 s. 5.