Richard Gilman, whose interesting history of the notion of decadence proves it to be more a carelessly applied epithet than an aesthetic, social, or political concept, traces the origins of the term back to the English dandyism of the Regency figure Beau Brummel, next to Jules Barbey d'Aurevilly, and thence to the poet Charles Baudelaire with whom
truly begins what has been called — and in some quarters aggressively proclaimed itself to be — the Decadence in France. Ironies bristle around this origin. He himself thought the word "vague" and too convenient, a cant term to be brandished by "ignorant pedagogues." Yet most of the phenomena it was and is supposed to describe can be traced to him: that is, he is found as the early object of their attribution. That side of "decadence" that is thought to traffic in the grotesque or the sexually bizarre or the freakish, in sensations of exquisite and sterile refinement, in languorous debauchery and cultivated eroticism ("It was Baudelaire," Paul Eluard wrote, "who first noticed the disturbing sensuality in the contrast between the white thighs of cancan dancers and their black stockings") — all this is, or is supposed to be, Baudelaire's province, his spiritual caliphate. He is the source, for those who want to think so, of the imagination gone awry, become sick, and losing its moral base; he is the organizer, it is said, of the modern imagination as cicerone to the perverse. [86-87]
Such a popular and long-lived characterization of the poet so beloved by Swinburne turns out to be, argues Gilman, "a superficial indictment, an obtuse one in fact" because it fails to take into account Baudelaire's particular "kind of spirituality" — one that recognizes a "sense of sin, of moral anxiety and incompleteness; these were conditions of experience which he knew to be largely absent from his age of secular complacency and which he therefore set himself to reintroduce" (87). He was, says Gilman, "the man who had raised the most fundamental question of all: what are we to do with our souls in an age that does not recognize their existence?"
The common idea of Baudelaire as decadent, particularly in the sense of being perverse, derives largely from the preface that Théophile Gautier wrote for the edition of Les Fleurs du mal published in 1868, the year after the poet's death. Gautier, says Gilman, "was the first to call him decadent," a term that had previously not been "used to characterize persons, and in fact, in applying it to the poet, Gautier was actually invoking the work, as when we speak, for example, of Shakespeare's qualities and mean the plays' rather than the man's. But the term would quickly slip away from its technical or literary use and come to spread its moral connotation over the man" (89). Gautier certainly encouraged readers to slide from text to author when he described Baudelaire's
morbidly rich tints of decomposition, the tones of mother-of-pearl which freeze stagnant waters, the roses of consumption, the pallor of chlorosis, the hateful bilious yellows, the leaden grey of pestilential fogs, the poisoned and metallic greens smelling of sulphide of arsenic . . . the bitumens blacked and browned in the depths of hell, and all that gamut of intensified colours, correspondent to autumn, and the setting of the sun, and the last hours of civilization. [89-90]
Swinburne's 1862 translations of poems from Les Fleurs du mal in large part produces what Gilman terms the "emotional and melodramatic side of Baudelaire's effect on English sensibility."
From then on "advanced" British consciousness, especially the literary side of it, was powerfully affected — though seldom with full comprehension of what it was undergoing — by the movement into hitherto suppressed regions of the psyche and imagination, the aggressive stance against bourgeois proprieties and dogmas and the turning of "negative" subjects into materials for art that had been so largely initiated in France and that had culminated in her so-called Decadence. 
Gilman, Richard. Decadence: The Strange Life of an Epithet. N.Y.: Farrar, Strauss and Giroux, 1979.
Last modified 21 February 2007