永世|えいせいeisei[0]heiban (flat)

1. nouneternity; perpetuity; immortality; permanence
2. nounlifetime title holder (in shogi, go, etc.)

• Synonym永遠えいえんNeutralThe most general everyday word, with emotional overtones, as in 「永遠の愛」. 「永世」 is largely confined to fixed terms like 「永世中立」.
• Synonym永久えいきゅうNeutralMore objective in feel and very widely used, as in 「永久保存」. 「永世」 appears mainly in fixed expressions for titles and the like.
• Synonym恒久こうきゅうWrittenA written, formal term for what endures unchanged over the long run, as in 「恒久平和」; 「永世」 is largely limited to set phrases.
• Synonym悠久ゆうきゅうWrittenEvokes long, distant stretches of time, typically of history or nature, as in 「悠久の歴史」. 「永世」 leans institutional.
• Synonym永代えいたいWrittenMeans permanence and lives mostly in fixed expressions like 「永代供養」 and 「永代橋」 — like 「永世」, largely a set-phrase word.
• Synonym長久ちょうきゅうWrittenFor long continuance, often in wishes such as 「武運長久」. 「永世」 leans toward institutional usage.
• Synonym永劫えいごうWrittenA Buddhist term for immensely long ages, as in 「未来永劫」. 「永世」 leans toward institutional and title usage.
• Synonym久遠くおんWrittenPoints to the remote past or future and shows up in religious and literary writing; more mystical in flavor than 「永世」.
• Synonym無窮むきゅうWrittenCovers the endlessness of time and space, as in 「天壌無窮」. 「永世」 is mostly a word of institutions or titles.
• Synonym未来永劫みらいえいごうWrittenA four-character expression meaning forever from now on, with strong emphasis. 「永世」 serves mainly as title and institutional vocabulary.
• Related千代ちよWrittenAn elegant word for a thousand years or long ages, heard in the anthem 「君が代」; it has a celebratory tone, while 「永世」 is institutional.
• Related万世ばんせいWrittenWritten 「万世」, it stresses enduring through all generations and appears in blessings; 「永世」 leans toward titles and institutions.
• Related万代ばんだいWrittenA rare written word for all generations to come, found chiefly in congratulatory language; in daily speech say 「永遠」.
• Related生々世々しょうじょうせぜWrittenA Buddhist phrase for endless rounds of rebirth, now very rare. 「永世」 carries no sense of reincarnation.
• Related永久とわNeutralMore objective in feel and very widely used, as in 「永久保存」. 「永世」 appears mainly in fixed expressions for titles and the like.
• Related常世とこよWrittenAn archaic word for what never changes, but also for the mythical land across the sea and the world of the dead — senses 「永世」 lacks.
• Related千古せんこWrittenCan point to the distant past, as in 「千古の昔」, as well as to eternity; 「永世」 speaks only of an endless stretch ahead.
• Related万古ばんこWrittenRefers to remote antiquity or time since long ago, as in 「万古不易」. 「永世」 never looks to the past and is institutional.
• Related万劫ばんごうWrittenA Buddhist term for an exceedingly long span of years, seldom used. 「永世」 leans toward institutional and title usage.
• RelatedエタニティNeutralA loanword for eternity, mostly seen in product names like the 「エタニティリング」 wedding band; everyday speech prefers 「永遠」.
• Related万劫末代まんごうまつだいWrittenA rare, emphatic four-character word for endless generations of descendants ahead. 「永世」 is more a term of institutions.
• AntonymひとときNeutralThe everyday antonym: a moment or short while, or some period in the past — the opposite of 「永世」's permanence.
• Antonym暫定ざんていWrittenA formal, written antonym: a provisional arrangement until things are settled, as in 「暫定措置」, versus 「永世」's permanence.

In everyday speech, 「永遠」 and 「永久」 are the default words for forever. 「永世」 itself turns up mostly in fixed expressions such as 「永世中立国」 and 「永世名人」. For a brief moment say 「ひと時」; for something provisional, 「暫定」.

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

1. 限かぎりなく続つづくこと。また、そのような世よ。