How to say "commonwealth" in Japanese

• 連邦れんぽうJLPT N1approx.Common
  nounFor 'commonwealth' meaning a union of states — the same idea as 'federation' — this is the word; it frames the commonwealth by its member states rather than by its form of government.
  アメリカ人あめりかじんは連邦れんぽう税ぜいと州税しゅうぜいの両方りょうほうを払はらっている。↗

  Americans pay both federal taxes and state taxes.
• 共和国きょうわこくCommon
  nounFor 'commonwealth' meaning a republic — a state without a monarch — this is the word; it fits when the English term points to the state's republican form of government rather than to a union of states.
  アメリカ合衆国あめりかがっしゅうこくは共和国きょうわこくである。↗

  The United States is a republic.

Choose 連邦 when 'commonwealth' refers to a union or federation of states joined under one government, and 共和国 when it refers to a republic — a state defined by its non-monarchical, republican form of government.

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

• 連邦れんぽうfederation (of states)
• 共和国きょうわこくrepublic
• コモンウェルスCommonwealth (of Nations)