Refers to the period when something or someone was at the peak of power, prosperity, or influence.
The golden age of the Roman Empire was around the 2nd century AD.
彼は全盛時代の勢いを失ってしまった。
He has lost the vigor of his prime.
全盛期 is more common and refers to the peak period itself, while 全盛時代 emphasizes the era or age.
Compound of 全盛 (zen-sei, 'full prosperity') and 時代 (jidai, 'era').