ADV is an abbreviation for Adjective Modifier, which is a type of word used to modify or describe a noun or pronoun. An adjective modifier can be used to provide more information about the noun or pronoun it modifies, such as size, color, shape, or age. For example, in the sentence 'The SMALL red CAR drove down the street, ' the word 'SMALL' is an adjective modifier that provides more information about the noun 'CAR.'