极端 (jí duān) can be an adjective ('extreme'), an adverb ('extremely'), or a noun ('extremes'). As an adverb, it is more formal and emphatic than 极 (jí) and often implies going beyond what is reasonable or normal. Unlike the other adverb synonyms (极其, 极为, 极度), 极端 also functions as a noun, e.g., 走极端 'go to extremes'.