The word decadence, which at first meant simply "decline" in an abstract sense, is now most often used to refer to a perceived decay in standards, morals, dignity, religious faith, honour, discipline, or skill at governing among the members of the elite of a very large social structure, such as an empire or nation state. 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 is 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 factors contributing to declinism include the reminiscence bump as well as both the positivity effect and negativity bias.

In literature, the Decadent movement—late nineteenth century fin de siècle writers who were associated with Symbolism or the Aesthetic movement—was first given its name by hostile critics. Later it was 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.