golden age; golden era
Refers to a peak period of prosperity, achievement, or cultural flourishing.
この国は1960年代に経済の黄金期を迎えた。
This country experienced a golden age of its economy in the 1960s.
ルネサンスは芸術の黄金期だった。
The Renaissance was a golden age of art.
Compound of 黄金 (おうごん, 'gold') and 期 (き, 'period'). The exact historical derivation is uncertain, but the meaning is transparent from its components.