noun
prime of life; era of national prosperity
Refers to a period of great flourishing, either for an individual (prime of life) or for a nation (prosperous era). Often used in historical or literary descriptions.
彼は盛時に多くの傑作を残した。
He produced many masterpieces in his prime.
この国は江戸時代に盛時を迎えた。
This country reached an era of prosperity during the Edo period.
Compound of 盛 (prosperous, flourishing) and 時 (time, period). The reading せいじ is the on'yomi for both characters.