1. moral or cultural decline as characterised by excessive indulgence in pleasure or luxury.

2. a process, condition, or period of deterioration or decline, as in morals or art; decay; turpitude.

3. often Decadence: A literary movement especially of late 19th-century France and England characterised by refined aestheticism, artifice, and the quest for new sensations.

Etymology: from Middle French < Mediaeval Latin dēcadentia, equivalent to Late Latin dēcadent- (stem of dēcadēns), present participle of dēcadere, “to fall away”.

[Yohey Horishita - The Great Gatsby]

I like the word ‘decadent,’ all shimmering with purple and gold … it throws out the brilliance of flames and the gleam of precious stones. It is made up of carnal spirit and unhappy flesh and of all the violent splendors of the Lower Empire; it conjures up the paint of the courtesans, the sports of the circus, the breath of the tamers of animals, the bounding of wild beasts, the collapse among the flames of races exhausted by the power of feeling, to the invading sound of enemy trumpets. The decadence is Sardanapalus lighting the fire in the midst of his women, it is Seneca declaiming poetry as he opens his veins, it is Petronius masking his agony with flowers.
—  Paul Verlaine

Alphonse Mucha, Salammbô, 1896.

The fin de siècle in France is known retrospectively as la Belle Époque – the Beautiful Era, the time anticipating the 1900 Exposition Universelle, the time before the War. When we peer through the mists of imaginative history, the era is illuminated by a magical glow. It was always a lush twilight, Montmarte was flooded with geniuses and bohemians, the cafés lit by those charming globe-shaped lamps, the cuisine was haute and absinthe flowed from the fountains, the scent of Grasset’s flowers and the tinkling of Debussy and Satie wafted through the air, all the actresses were Sarah Bernhardt, all the can-can dancers were Toulouse-Lautrecs, all the men were Sem’s quipping dandies sporting green carnations to match their Pernod and all the wine-warm laughing ladies in the cabarets had hair made of twining Mucha whiplash lines, all their jewelry was Lalique, all the furniture was Majorelle and unfurled itself in organic floral curves, even all the posters pasted on the buildings were bright and beautiful Art Nouveau advertising the gay nightlife of the City of Light.

This was the same city Jean Lorrain called the Poisoned City. This beautiful era was also the reign of the Decadence, languidly awaiting the Apocalypse on the deathbed of history, disgusted and exhausted by all the crass gaiety, seeing in every woman a femme fatale and in every man a syphilitic Sodomite, and fleeing into dreams, drugs or the occult. We see the two faces of the fin de siècle meet at times, in Mucha for example, here taking the incense, flowers and peacock feathers of Symbolism’s favorite Flaubert heroine and rendering them in the bold-lined, glowing, graphic style that assured his fame as a master Art Nouveau confectioner.