ad valorem

type of customs duties based on the market value of goods at the original place of shipment, 1711, Modern Latin, "(in proportion) to the value," from ad "to" (see ad-) + Late Latin valorem, accusative of valor "value" (see value (n.)). Sometimes abbreviated ad val.

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Definitions of ad valorem from WordNet

ad valorem (adv.)
in proportion to the estimated value of the goods taxed;
the goods were taxed ad valorem
Synonyms: ad val