Intensifier is a linguistic term (but not a proper lexical category) for a modifier that makes no contribution to the propositional meaning of a clause but serves to enhance and give additional emotional context to the word it modifies. Intensifiers are grammatical expletives, specifically expletive attributives (or, equivalently, attributive expletives or attributive-only expletives; they also qualify as expressive attributives), because they function as semantically vacuous filler. Characteristically, English draws intensifiers from a class of words called degree modifiers, words that quantify the idea they modify. More specifically, they derive from a group of words called adverbs of degree, also known as degree adverbs. However, when used grammatically as intensifiers, these words cease to be degree adverbs, because they no longer quantify the idea they modify; instead, they emphasize it emotionally. By contrast, the words moderately, slightly, and barely are degree adverbs, but not intensifiers. The other hallmark of prototypical intensifiers is that they are adverbs which lack the primary characteristic of adverbs: the ability to modify verbs. Intensifiers modify exclusively adjectives and adverbs. However, this rule is insufficient to classify intensifiers, since there exist other words commonly classified as adverbs that never modify verbs but are not intensifiers, e.g. questionably.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/intensifier