极 appears in 极了 (jí le, extremely — used after adjectives), 积极 (jījí, positive/active), 太极 (tàijí, tai chi), 北极 (Běijí, North Pole), 南极 (Nánjí, South Pole).
grammar
The pattern 'adjective + 极了' is a very common way to express superlatives: 好吃极了 (incredibly delicious), 漂亮极了 (extremely beautiful).
