Decadence was a period in literature that had its origin in the France of the 1890s. It was influenced by and was a development of Romanticism and Naturalism. It evolved in to Symbolism and from there to Surrealism.
Major themes of Decadent literature are death, decay, perversion and artificiality.
Some of the great Decadent writers are J.K. Huysmans who wrote A rebours (translated as Against Nature) and Villiers de l'Isle-Adam who wrote Axel.