永久 is an adjective meaning 'permanent, everlasting', used to describe a state or condition that lasts indefinitely. Unlike the adverb 永远 (yǒng yuǎn, 'forever'), which modifies verbs, 永久 modifies nouns (e.g., 永久的和平). It is more formal and absolute than 长期 (cháng qī, 'long-term'), which implies a finite duration, and more concrete than 永恒 (yǒng héng, 'eternal'), which is poetic or philosophical.