What is Ad Valorem?

Legal Definition
A Latin term meaning "according to [the] value [of something]". The term is used in taxation to designate taxes levied against property, real or personal, at a certain rate based upon the property's value.

An ad valorem tax is levied according to the value of the property as determined by an assessment or appraisal. Assessment on a regular basis is a common characteristic of an ad valorem tax.
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
According to value. Duties are either ad valorem or specific; the former when the duty is laid in the form of a percentage on the value of the property; the latter where it is imposed as a fixed sum on each article of a class without regard to its value. The term ad valorem tax is as well defined and fixed as any other used in political economy or legislation, and simply means a tax or duty upon the value of the article or thing subject to taxation. Bailey v. Fuqua, 24 Miss. 501; Pingree v. Auditor General, 120 Mich. 95, 78 N. W. 1025, 44 In IU A. 679.
-- Black's Law Dictionary
Legal Definition
According to value.
-- Ballentine's Law Dictionary