正当 (zhèng dàng) means 'reasonable, fair, proper' — it emphasizes that something is morally or legally justifiable, not just logically sound. It is broader than 合理 (hé lǐ, 'rational, sensible') and focuses on propriety rather than impartiality like 公正 (gōng zhèng, 'just, equitable'). Use it for actions, reasons, or means that are rightful and acceptable.