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height of prosperity
この歌手は今が全盛期で人気絶頂の状態にある。
This singer is at their prime and at the peak of popularity.
ローマ帝国の全盛期は紀元前後だと言われている。
The heyday of the Roman Empire is said to be around the time of Christ.
全盛を極めた王朝もいつかは衰退するものなのだ。
Even dynasties at their peak will eventually decline.