noun
phase of decline; ebb; twilight
Refers to a period when something is losing strength, influence, or vitality. Often used in historical, economic, or biological contexts.
ローマ帝国は衰退期に入っていた。
The Roman Empire had entered a phase of decline.
その産業は衰退期を迎えている。
That industry is facing a period of decline.
Antonym: 全盛期 means 'heyday' or 'golden age', the peak period, in contrast to 衰退期's phase of decline.
Compound of 衰退 (decline) + 期 (period). The exact historical derivation is uncertain, but the meaning is transparent from its components.