In linguistics, a modifier is an optional element in phrase structure or clause structure which modifies the meaning of another element in the structure. For instance, the adjective "red" acts as a modifier in the noun phrase "red ball", providing extra details about which particular ball is being referred to. Similarly, the adverb "quickly" acts as a modifier in the verb phrase "run quickly". Modification can be considered a high-level domain of the functions of language, on par with predication and reference.
Modifiers may come either before or after the modified element (the head), depending on the type of modifier and the rules of syntax for the language in question. A modifier placed before the head is called a premodifier; one placed after the head is called a postmodifier.
For example, in land mines, the word land is a premodifier of mines, whereas in the phrase mines in wartime, the phrase in wartime is a postmodifier of mines. A head may have a number of modifiers, and these may include both premodifiers and postmodifiers. For example:
that nice tall man from Canada whom you met
In this noun phrase, man is the head, nice and tall are premodifiers, and from Canada and whom you met are postmodifiers.
Notice that in English, simple adjectives are usually used as premodifiers, with occasional exceptions such as galore (which always appears after the noun, coming from Irish in which most adjectives are postmodifiers) or the adjectives immemorial and martial in the phrases time immemorial and court martial (the latter comes from French, where most adjectives are postmodifiers). Sometimes placement of the adjective after the noun entails a change of meaning: compare a responsible person and the person responsible, or the proper town (the appropriate town) and the town proper (the area of the town as properly defined).
It is possible in English (and other languages) for a modifier to be separated from its head by other modifiers, making the phrase discontinuous, as in The man here whom you bumped into in the street yesterday, where the relative clause who.