Decadence was a late-19th-century movement emphasizing the need for sensationalism, egocentricity, and bizarre, artificial, perverse, and exotic sensations and experiences. By extension, it may refer to a decline in art, literature, science, technology, and work ethics, or (very loosely) to self-indulgent behavior.

Usage of the term sometimes implies moral censure, or an acceptance of the idea, met with throughout the world since ancient times, that such declines are objectively observable and that they inevitably precede the destruction of the society in question; for this reason, modern historians use it with caution. The word originated in Medieval Latin (dēcadentia), appeared in 16th-century French, and entered English soon afterwards. It bore the neutral meaning of decay, decrease, or decline until the late 19th century, when the influence of new theories of social degeneration contributed to its modern meaning.

The idea that a society or institution is declining is called declinism. This may be caused by the predisposition, caused by cognitive biases such as rosy retrospection, to view the past more favourably and future more negatively. Declinism has been described as "a trick of the mind" and as "an emotional strategy, something comforting to snuggle up to when the present day seems intolerably bleak." Other cognitive factors contributing to the popularity of declinism may include the reminiscence bump as well as both the positivity effect and negativity bias.

In literature, the Decadent movement began in France's fin de siècle intermingling with Symbolism and the Aesthetic movement while spreading throughout Europe and the United States. The Decadent title was originally used as a criticism but it was soon triumphantly adopted by some of the writers themselves. The Decadents praised artifice over nature and sophistication over simplicity, defying contemporary discourses of decline by embracing subjects and styles that their critics considered morbid and over-refined. Some of these writers were influenced by the tradition of the Gothic novel and by the poetry and fiction of Edgar Allan Poe.

Decadence is a popular criticism of the culture of the later Roman Empire's elites, seen also in much of its earlier historiography and 19th and early 20th century art depicting Roman life. This criticism describes the later Roman Empire as reveling in luxury, in its extreme characterized by corrupting "extravagance, weakness, and sexual deviance", as well as "orgies and sensual excesses".^{[excessive citations]}

According to Professor Joseph Bristow of UCLA, decadence in Rome and the Victorian-era movement are connected through the idea of "decadent historicism." In particular, decadent historicism refers to the "interest among…1880s and 1890s writers in the enduring authority of perverse personas from the past" including the later Roman era. As such, Bristow's argument references how Heliogabalus, the title subject of Simeon Solomon's painting Heliogabalus, High Priest of the Sun (1866), was "a decadent icon" for the Victorian movement. Bristow also notes that "[t]he image [of the painting] summons many qualities linked with fin-de-siècle decadence [alongside his]…queerness[,]" thus "inspir[ing] late-Victorian writers [as]…they…imagine anew sexual modernity."

Heliogabalus is also the subject of The Roses of Heliogabalus (1888) by Sir Lawrence Alma-Tadema, which, according to Professor Rosemary Barrow, represents "the artist['s]…most glorious revel in Roman Decadence." To Barrow, "[t]he authenticity of the [scene]…perhaps had little importance for the artist[, meaning that] its appeal is the entertaining and extravagant vision it gives of later imperial Rome." Barrow also makes a point to mention "that Alma-Tadema’s Roman-subject paintings [tend to]…make use of historical, literary and archaeological sources" within themselves. Thus, the presence of roses within the painting as opposed to the original "'violets and other flowers'" of the source material emphasizes how "the Roman world…h[eld] extra connotations of revelry and luxuriant excess" about them.

Decadence was the name given to a number of late nineteenth-century writers who valued artifice over the earlier Romantics' naïve view of nature. Some of them triumphantly adopted the name, referring to themselves as Decadents. For the most part, they were influenced by the tradition of the Gothic novel and by the poetry and fiction of Edgar Allan Poe, and were associated with Symbolism and/or Aestheticism.