一世|いっせいissei[1]atamadaka (head-high)

1. noungeneration; lifetime
2. nounthe age; the day
3. noun, noun (suffix)the first (e.g. Elizabeth I)
   １６０３年ねん、ジェームズ一世いっせいが政権せいけんの座ざについたとき、フットボールは再ふたたび許可きょかされたのです。↗

   In 1603, when King James I came into power, football was allowed again.
4. nounissei; first-generation Japanese (or Korean, etc.)

• Synonym一生いっしょうNeutralThe everyday word for one's whole life. 「一世」 covers a generation, a lifetime, or the first of a line, as in 「一世一代」 and 「ナポレオン一世」.
• Related人生じんせいNeutralOne's life as lived — its course and fortunes. Unlike 「一世」, it never means a generation or the first of a line.
• Related生涯しょうがいWrittenOne's whole life, in written or formal prose, as in 「生涯学習」.
• Related一期いちごWrittenA lifetime; in modern usage it mostly turns up in 「一期一会」 and is rarely used on its own.
• RelatedライフNeutralA loanword for life or living, used mostly in compounds such as 「ライフスタイル」.
• Related一生涯いっしょうがいNeutralThe whole span of one's life; carries more weight than 「一生」.
• Related畢生ひっせいWrittenOne's whole life, in written prose, mostly in set phrases like 「畢生の大作」.

「一世」 mostly appears in fixed phrases such as 「一世一代の大勝負」 and 「一世を風靡する」, or for first-generation immigrants and numbered rulers like Elizabeth I. For a whole life, say 「一生」; in formal prose, 「生涯」.

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

1. （いっせ、いっせい、ひとよ）一生いっしょう。
2. （いっせ、いっせい）一いち代だい。
3. （いっせい）初代しょだい。一いち代目だいめ。
4. （いっせい）その時代じだい。ある時期じき。