A prosperous or peak period when something is flourishing; the heyday of an era or activity.
その王朝は盛期に多くの文化が栄えた。
Many cultural achievements flourished during the dynasty's heyday.
バブル経済の盛期には不動産の値段が急騰した。
During the peak of the bubble economy, real-estate prices skyrocketed.
学びの盛期は若い時だと言われている。
It is said that the prime period for learning is when one is young.