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In grammar, a modifier is an optional element in phrase structure or clause structure. Typically the modifier can be removed without affecting the grammar of the sentence.
For example, in the English sentence This is a red ball, the adjective red is a modifier, modifying the noun ball. Removal of the modifier would leave This is a ball, which is grammatically correct and equivalent in structure to the original sentence. These concepts are often distinguished from complements and arguments, which may also be considered dependent on another element, but are considered an indispensable part of the structure.