In
grammar, a
modifier is an optional element in
phrase structure or
clause structure. A modifier is so called because it is said to
modify (change the meaning of) another element in the structure, on which it is dependent. 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.