Noun, Suru verb, Intransitive verb
decline; degeneration; decay; waning; ebbing
４７５年はローマ帝国の「衰退」を示す年ではあっても、「滅亡」を意味する年ではありません。Although 475AD is the year that shows the 'decline' of the Roman Empire, it is not the year of its 'fall'.
Also written 衰頽【すいたい】