The term “decadence” had been in cultural use throughout Europe since at least the Middle Ages and generally refers to a prolonged state of societal moral decline. Its aesthetic uses, however, emerged in 19th century France. In its aesthetic connotation, decadence described a set of anti-Romantic approaches that denigrated the natural and moral in favour of the unnatural, the grotesque, and the taboo. Though often used a critique against artists, decadence was first used in a positive light by Gaultier in describing Baudelaire’s book of poems Les Fleurs du mal in 1868.

The Decadent movement was imported to the Victorians by the likes of Swinburne and Wilde, and coalesced with the Aesethic movement occurring in Britain. For instance, Wilde’s Decadent tragedy Salome—originally written in French while Wilde was living in Paris—used Mallarmé’s Hérodiade and Flaubert’s Hérodias (both retellings of the biblical story of the execution of John the Baptist) as its source material. Meanwhile, Swineburne was famously influenced by Baudelaire and was the first English critic to engage with Baudelaire’s work.

Sources:

Buchanan, Ian. "decadence." A Dictionary of Critical Theory. Oxford University Press, Oxford Reference, 2018. https://www.oxfordreference.com/view/10.1093/acref/9780198794790.001.0001/acref-9780198794790-e-164. 

Cohen, Phillip K. "How the Decadents differ from the Aesthetes and the Aesthetic Movement." The Victorian Web, Dec 12, 2012. http://www.victorianweb.org/decadence/cohen.html