永恒 (yǒnghéng) is an adjective meaning 'eternal, everlasting' — it describes something that lasts forever, often in a poetic or grand sense. It differs from the adverb 永远 (yǒngyuǎn 'forever'), which modifies verbs, and from 永久 (yǒngjiǔ 'permanent'), which is more concrete and less emotional. 永恒 is the most elevated choice, common in literature and philosophy.