peak of maturity; full maturity; zenith
Refers to the stage when something (a person, culture, art, etc.) has reached its fullest development or ripest state. Often used in literary or formal contexts.
His art has reached its peak of maturity.
A culture at its zenith sometimes shows signs of decadence as well.
全盛期 means 'heyday' or 'golden age', focusing on the period of greatest prosperity or power, without the nuance of overripeness.
Compound of 爛熟 (らんじゅく, 'overripeness; full maturity') + 期 (き, 'period; stage'). 爛熟 itself is from 爛 (らん, 'to rot; to be overripe') and 熟 (じゅく, 'to ripen').