无比 (wú bǐ) means 'unparalleled, matchless' and is often used as an adverb before adjectives to mean 'extremely' (e.g., 无比激动 'incredibly excited'). 无敌 (wú dí) means 'invincible, unmatched' and describes a person or thing that cannot be defeated in competition. While both convey being without equal, 无比 focuses on degree and modifies adjectives, whereas 无敌 is used for unbeatable entities.