Whether in reference to chocolate cake for breakfast or wild all-night parties, decadence means extravagance, luxury, and self-indulgence with a sense of moral decline.
The Latin root of decadence means "to fall down," and this may help you understand the full sense of the word. Decadence is not simply a synonym for excess; it also suggests that one's morals have gone down. The word is usually negative but not always. When hostile critics called a group of French writers and artists in the late 1800s decadents, these poets and painters embraced the label. When a pastry chef calls her creation Chocolate Decadence, she is promoting a mildly guilty pleasure, not announcing the end of Western civilization.