noun
decline; waning fortunes; decay
Refers to a loss of power, influence, or prosperity, often used for nations, organizations, or individuals. Somewhat formal and literary.
その政党は退勢を挽回できなかった。
The political party could not recover from its decline.
退勢にある企業を立て直すのは容易ではない。
It is not easy to turn around a company in decline.
衰退 is a more common and general word for decline or decay, used for both concrete and abstract things. 退勢 is more literary and often implies a loss of momentum or power.
没落 refers to a fall from prosperity or status, often with a sense of ruin. 退勢 focuses on the waning trend rather than the final collapse.
Compound of 退 (retreat, decline) and 勢 (power, force). The combination conveys the idea of power receding or waning.