悠久|ゆうきゅうyuukyuu[0]heiban (flat)

1. no-adjective, na-adjectiveeternal; everlasting; perpetual; immemorial

• Synonym永遠えいえんNeutralThe most general word for eternity, carrying emotional weight as in 「永遠の愛」; unlike 悠久 it does not specifically evoke the long sweep of history or nature.
• Synonym永久えいきゅうNeutralObjective permanence, as in 「永久保存」 or 「永久歯」; 悠久 adds a sense of historical depth that 永久 lacks.
• Synonym永世えいせいWrittenWritten-style word seen mostly in fixed phrases such as 「永世中立」; it is not used to describe history or nature.
• Synonym永代えいたいWrittenWritten-style word for permanence, found chiefly in fixed phrases like 「永代供養」 or 「永代橋」.
• Synonym恒久こうきゅうWrittenWritten-style word for lasting permanence, as in 「恒久平和」; it leans toward policy and ideals.
• Synonym長久ちょうきゅうWrittenWritten-style word for long continuance, as in 「武運長久」; mostly used in expressions of good wishes.
• Synonym万世ばんせいWrittenWritten-style word for lasting through all generations, used in felicitations; 「万世」 is close to 悠久 in feel.
• SynonymエタニティNeutralLoanword for eternity, often seen in product names such as wedding rings; it lacks the historical feel of 悠久.
• Related限りないかぎりないNeutralBoundless, with emotional color as in 「限りない愛」; it does not specifically point to the remoteness of time.
• Related果てしないはてしないNeutralEndless in space or time alike, as in 「果てしない宇宙」 or 「果てしない時間」; 悠久 is said of time only, a vast span of ages.
• Related不滅ふめつWrittenWritten-style word for undying, as in 「不滅の記録」; it stresses never perishing, where 悠久 stresses remoteness in time.
• Related無限むげんNeutralInfinity in a mathematical or abstract sense; it carries none of the historical depth of 悠久.
• Related不変ふへんNeutralUnchanging, as in 「不変の真理」; it stresses the absence of change, while 悠久 stresses length of time.
• Related永久とわNeutralObjective permanence, as in 「永久保存」 or 「永久歯」; 悠久 adds a sense of historical depth that 永久 lacks.
• Related永続的えいぞくてきWrittenWritten-style word for something lasting on, as in 「永続的な平和」; institutional in flavor, and not said of history.
• Related果てしがないはてしがないNeutralColloquial variant meaning boundless, often as 「果てしがない話」; it is not used of history.
• Related常しなえとこしなえWrittenArchaic, literary word for eternity, found in waka and classical literature; more lyrical than 悠久.
• Related恒久的こうきゅうてきWrittenWritten-style word for permanent, as in 「恒久的な対策」; leans toward policy, and is not said of history.
• Related千古不易せんこふえきWrittenLiterary four-character word for what never changes through the ages, said chiefly of truths; it stresses unchangeability, not remoteness.
• Related無窮むきゅうWrittenWritten-style word for infinitude, as in 「天壌無窮」; it covers space as well as time and has a solemn ring.
• RelatedパーマネントNeutralLoanword for permanent; in everyday speech it usually means a perm, 「パーマ」, and has nothing to do with history.
• Related不易ふえきWrittenWritten-style word for constancy, known from haiku theory in 「不易流行」; it stresses what never changes.
• Related万代不易ばんだいふえきWrittenLiterary four-character word for remaining unchanged through all generations, used of institutions and principles.
• Related持続性じぞくせいNeutralDurability or persistence, an academic and business term; it carries no sense of history.
• Related尽きないつきないNeutralInexhaustible, as in 「話が尽きない」; it focuses on a supply or quantity that never runs out.
• Related万古不易ばんこふえきWrittenLiterary four-character word for what stays unchanged for all ages, said of truths and principles.
• Related幾久しいいくひさしいWrittenWritten-style adjective for lasting long and unchanged, used in wedding congratulations as 「幾久しく」; it carries a blessing.
• Related万代ばんだいWrittenWritten-style word for all generations to come, used in felicitations; more of a wish than 悠久.
• Antonym一瞬いっしゅんNeutralAn instant, the blink of an eye; the direct opposite of the long stretch of time that 悠久 describes.
• Antonym刹那せつなWrittenWritten-style word for the briefest instant, originally a Buddhist term, as in 「刹那的」; the opposite of 悠久.
• Antonym短命たんめいNeutralA short life, or short-lived, as in 「短命政権」; the opposite of 悠久.

For eternity in general, say 「永遠」 or 「永久」. Use 「悠久」 when history or nature stretches back through the ages, as in 「悠久の歴史」 or 「悠久の大地」. For a fleeting instant, use 「一瞬」 or 「刹那」. Among the rest, 永続的 and 恒久的 fit policy talk, 不変 and 不易 stress unchangeability, and words like 万代, 長久 and 幾久しい are literary terms of blessing.

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

1. 果はてしなく長ながく続つづくこと。またはその時間じかん。