自然 (zì rán) as a noun means 'nature, the natural world' (like 自然界, but shorter and more common). As an adjective it means 'natural, unaffected' — 天然 (tiān rán) is for physical 'natural vs. artificial' (e.g., 天然材料), while 自然 describes a person's manner or a process. As an adverb it means 'naturally, of course', overlapping with 当然 (dāng rán) but 自然 emphasizes an inevitable result, 当然 a logical certainty.