Literally: according to value.
(general) Any charge, tax, or duty that is applied as a percentage of value.
(taxation) A tax calculated on the value of the property subject to the tax.
(shipping) A freight rate set at a certain percentage of the declared value of an article.
(U.S. Customs) Ad valorem duty. A duty assessed as a percentage rate or value of the imported merchandise. For example, 5% ad valorem. See compound rate of duty; specific rate of duty; tariff.