Grammatical modifier

The word modifier applies to either the adjective or the adverb in a sentence. An adjective is not actually required but helps modify and limit the extent of the meaning of a noun or pronoun, while an adverb helps modify a verb in a similar way. Thus modifiers, though not actually necessary help define the meaning and are attributes of nouns, pronouns or verbs. A modifier is also called a qualifier.

The adjective "green" in "a green tree" modifies and thus limits the meaning of the noun "a tree" in that it cannot be "a deciduous tree in winter", as the adverb "kindly" modifies the past tense of the verb "let" in "she kindly let me borrow her scissors".
An adverb may also modify an adjective, such as in "abjectly poor".

Another way of defining a modifier is that it, the adjective or adverb, is more dependent on the part of the sentence it modifies, namely the noun or verb. The noun is independent and may exist on its own without being modified: "the tree" but may be modified by "green", or the verb "work" in "I work (every day)" by the adverb "hard" in "I work hard (every day)".

In compound nouns, the first of the two words so combined functions as a modifier, such as "elementary" in "elementary school", "mountain" in "mountain bike", etc.