永久 (yǒng jiǔ) means 'permanent' or 'forever' and functions primarily as an adjective or adverb in Chinese. It combines two characters: 永 (yǒng), meaning 'eternal' or 'endless', and 久 (jiǔ), meaning 'long time' or 'for a long duration'. Together, they intensify the sense of duration — not just 'a long time', but 'without end'. This word emphasizes unchanging continuity and is commonly used in formal, written, or official contexts — such as legal documents, product warranties, policy statements, or solemn declarations.

Unlike colloquial alternatives like 一直 (yīzhí, 'all along') or 总是 (zǒngshì, 'always'), 永久 carries weight and finality. It rarely appears in casual speech but frequently occurs in phrases like 永久有效 (yǒngjiǔ yǒuxiào, 'permanently valid'), 永久保存 (yǒngjiǔ bǎocún, 'permanently save'), or 永久失去 (yǒngjiǔ shīqù, 'permanently lose'). Because it implies irrevocability, it’s often paired with verbs indicating change, loss, or establishment.