Single-word modifier

From Teflpedia

A single-word modifier is a single-word phrase that is a modifier, i.e. adjectives and adverbs.

The word modifier by itself usually refers to a modifier phrase, which can be either a single-word modifier or, if a multi-word phrase, a multi-word modifier. This, somewhat confusingly, is a different system to that employed for modifiers, where an adjective is a single-word adjective phrase, and an adverb is a single-word adverb phrase.