A compound modifier (also called a compound adjective, phrasal adjective, or adjectival phrase) is a compound of two or more attributive words: That is, more than one word that together modify a noun. Compound modifiers are grammatically equivalent to single-word modifiers, and can be used in combination with other modifiers. Note that in the preceding sentence, "single-word" is itself a compound modifier.

The constituent words of compound modifiers need not be adjectives; combinations of nouns, determiners, and other parts of speech are also common: man-eating (shark) and one-way (street).

The punctuation of compound modifiers in English depends on their grammatical role. Attributive compounds—modifiers within the noun phrase—are typically hyphenated, whereas the same compounds used as predicates will typically not be (if they are temporary compounds), unless they are permanent compounds attested as dictionary headwords.

Compound adjectives

Compound adjectives, like normal adjectives, modify noun phrases. In the phrases heavy metal detector and heavy-metal detector, the latter is a compound adjective because the modifier is made of two words used in combination.