Why is the ninth month called September?
1540s, from Middle French décadence (early 15c.), from Medieval Latin decadentia "decay," from decadentem (nominative decadens) "decaying," present participle of decadere "to decay," from Latin de- "apart, down" (see de-) + cadere "to fall" (see case (n.1)). Used of periods in art since 1852, on French model.
a period of decline or deterioration of art or literature that follows an era of great achievement. Examples include the Silver Age of Latin literature, which began about AD 18 following the end of the Golden Age, and the Decadent movement at the end of the 19th century in France and England.