关键 (guān jiàn) can be a noun ('key point, crux') or an adjective ('key, crucial'). Unlike 钥匙 (yào shi), which is only a noun and means a literal or metaphorical 'key' (e.g., key to a door or success), 关键 focuses on the decisive factor in a situation. Compared to 重点 (zhòng diǎn, 'main point, focus'), 关键 emphasizes what is most critical or determining, not just the most important among many.