Antonym: 全盛期 means 'heyday' or 'golden age', the peak period, in contrast to 衰退期's phase of decline.
全盛期 is the period of peak strength or popularity, while 衰退期 is the later period of decline.
衰退期 refers to a period of decline, the opposite of 最盛期, which is the period of peak prosperity or influence.
Compound of 衰退 (decline) + 期 (period). The exact historical derivation is uncertain, but the meaning is transparent from its components.