永久|えいきゅうeikyuu[0]heiban (flat)

1. na-adjective, no-adjectiveeternity; permanence; perpetuity
   彼かれは永久えいきゅうに日本にっぽんを離はなれた。↗

   He left Japan for good.
2. nounEikyū era (1113.7.13-1118.4.3)

• Synonym永遠えいえんNeutralEternity with a romantic, lyrical feel, as in 「永遠の愛」; 「永久」 is the more objective, institutional choice, as in 「永久歯」 and 「永久保存」.
• Synonym永世えいせいWrittenWritten word for eternity across the ages, as in 「永世中立国」 and 「永世名人」.
• Synonym永代えいたいWrittenMeans permanence, but mostly in fixed expressions such as 「永代供養」.
• Synonym恒久こうきゅうWrittenNoun for lasting permanence, as in 「恒久平和」; a written-language term.
• Synonym長久ちょうきゅうWrittenWritten word for long continuance, as in 「武運長久」; often in prayers and well-wishes.
• Synonym不滅ふめつWrittenWritten word for immortal, indestructible things, as in 「不滅の記録」; stresses never perishing.
• Synonym万世ばんせいWrittenAlso an adverb for all eternity; found in congratulatory addresses and stone inscriptions, not everyday speech.
• Synonym悠久ゆうきゅうWrittenFor the long sweep of history, as in 「悠久の歴史」; usually looks back into the past.
• Synonym無窮むきゅうWrittenWritten language for endlessness, applying to time or space alike, as in 「天壌無窮」.
• Synonym永劫えいごうWrittenWritten, Buddhist-flavoured word for immensely long ages; mostly seen in the set phrase 「未来永劫」.
• Synonym久遠くおんWrittenWritten word for the immensely remote in time, past or future, with a religious tinge.
• Synonym万劫ばんごうWrittenBuddhist term for endless stretches of time; rare in modern usage.
• Synonym未来永劫みらいえいごうWrittenFour-character phrase for forever from now on, usable in positive or negative statements, as in 「未来永劫許さない」; carries great weight.
• SynonymエタニティNeutralLoanword for eternity, seen mostly in product names such as 「エタニティリング」.
• Related限りないかぎりないNeutralAdjective for boundlessness, stressing range or degree rather than time.
• Related果てしないはてしないNeutralAdjective for having no end, as in 「果てしない宇宙」; the focus falls on vast space or a drawn-out process, whereas 「永久」 centres on time.
• Related無限むげんNeutralFor limitless quantity, space or time; 「永久」 applies only to endless time.
• Related不変ふへんNeutralFor being unchanging, stressing an unaltered state rather than long duration, as in 「不変の真理」.
• Related永久とわWrittenThe elegant reading 「とわ」, found in songs and poetry; the reading 「えいきゅう」 is the objective one.
• RelatedとこしえWrittenArchaic, elegant word for unchanging eternity, seen mainly in waka poetry and song lyrics.
• Related果てしがないはてしがないCasualColloquial twin of 「果てしない」 for things with no end; about boundless space or process, while 「永久」 is about time.
• Related永続的えいぞくてきWrittenWritten, expository word for lasting a long time, as in 「永続的な関係」.
• Related常しなえとこしなえWrittenAntiquated, elegant way of saying unchanging eternity; rarely used today.
• Related恒久的こうきゅうてきWrittenFor lasting, unchanging states, as in 「恒久的な平和」; leans political, found in formal writing.
• Related千古不易せんこふえきWrittenFour-character idiom for what never changes through the ages, as in 「千古不易の真理」.
• Related万代ばんだいWrittenWritten word for all generations to come, as in 「万代不易」; often in celebratory contexts.
• Related生々世々しょうじょうせぜWrittenBuddhist term for through all eternity, life after life; rare in modern usage.
• RelatedパーマネントNeutralLoanword for permanent, but most often means a perm, as in 「パーマ」.
• AntonymひとときNeutralA moment or a stretch of time, the opposite of 「永久」 and its unbroken continuance.
• Antonym一瞬いっしゅんNeutralAn instant, the blink of an eye; contrasts with 「永久」 in sheer length of time.

Use 「永久」 for objective, institutional permanence, as in 「永久保存」. For lyrical eternity choose 「永遠」; for limitless extent 「無限」; for what never perishes 「不滅」; for the long sweep of history 「悠久」. Poetic forms like 「とわ」 and 「常しえ」 belong to songs and formal prose, while 「永続的」 and 「恒久的」 suit formal discussion of lasting arrangements. For the briefest spans, 「一瞬」 is the opposite.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. （えいきゅう）ある時点じてんから限かぎりなく続つづくこと。
2. 日本にっぽんの元号げんごうの一ひとつ。天永てんえいの次つぎで、元永げんえいの前まえ。1113年ねん8月がつ25日かから1118年ねん4月がつ25日かまでの期間きかんのこと。
3. 日本人にほんじんの姓せいのひとつ。