Decadence can refer to a personal trait, or to the state of a society (or segment of it). Used to describe a person's lifestyle. Concise Oxford Dictionary: "a luxurious self-indulgence". Oscar Wilde gave a curious definition: "Classicism is the subordination of the parts to the whole; decadence is the subordination of the whole to the parts."
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, and then the name was triumphantly adopted by some writers themselves. These "decadents" relished artifice over the earlier Romantics' naive view of nature (see Jean-Jacques Rousseau). Some of these writers were influenced by the tradition of the Gothic novel and by the poetry and fiction of Edgar Allan Poe.
In a social context, the word 'decadent' is often used to describe corrosive decline due to a perceived erosion of moral traditions (A society that discards unnecessary and outmoded values would not be considered decadent, although perceptions of "unnecessary and outmoded" significantly vary). Due to arguments over the nature of morality, whether a society is decadent or not is a matter of debate.