A general word for a decline in cultural standards that was adopted to name an aesthetic style that emerged in the latter part of the 19th century in Europe. In the first instance decadence was, in spite of its subject matter which tends to recall ancient myths and supernatural tales, a kind of Avant-garde, or modernism inasmuch as it marked a break with the established aesthetic of its own time. Indeed, in his book on postmodernism, Fredric Jameson describes decadence as a kind of premonition. The first use of the word ‘decadence’ in this aesthetic sense occurred in Théophile Gautier's 1868 preface to Charles Baudelaire's collection of poems Les Fleurs du Mal (1857). However the work that has come to be regarded as the quintessential example of decadence is Joris-Karl Huysman's À rebours (1884), translated as Against Nature (1926), which interestingly enough is also regarded as one of the more important examples of naturalism. Decadence was also a crucial concern for Friedrich Nietzsche, although in his case it was something that philosophy had to figure out how to overcome.
M. Calinescu Five Faces of Modernity (1987).