Demi-monde is French for "half-world". The term derives from a play called Le Demi-Monde, by Alexandre Dumas fils, published in 1855. The play dealt with the way that prostitution at that time threatened the institution of marriage. The demi-monde was the world occupied by elite men and the women who entertained them and whom they kept, the pleasure-loving and dangerous world Dumas immortalized in the 1848 novel La Dame aux Camélias and its many adaptations. Demimondaine became a synonym for a courtesan or a prostitute who moved in these circles—or for a woman of social standing with the power to thumb her nose at convention and throw herself into the hedonistic nightlife. A woman who made that choice would soon find her social status lost, as she became "déclassée". The 1958 film Gigi, based on a 1944 novella by Colette, vividly portrays the world of the demimonde near the end of its existence. Gigi's Aunt Alicia, a legendary courtesan now enjoying a wealthy retirement, trains her teenage niece in elegant manners and deportment and the value of jewels and tries to stir her interest in fashion, in order to prepare her for life in the demimonde, pleasing the gentlemen who will provide her with the means to live comfortably.

For the men, the high life of the demimonde was isolated from the other world of wives and families and duties (if any). It embraced heavy drinking, drug use, gambling, attending the theatre and ballet and horse races, the pursuit of high fashion in every aspect of life—and, of course, sexual promiscuity. Lavish spending led to indebtedness; promiscuity, in the worst scenario, led to disease.

Historically, the height of the demimonde was encapsulated by the period known in France as La Belle Époque (1871–1914), from the end of the Franco-Prussian War to the beginning of World War I.

The twentieth century brought the rise of the New Woman, changing economies and social structures, as well as changing fashions and social mores, particularly in the aftermath of World War I. Prostitution and the keeping of mistresses did not disappear, but the label demimondaine became obsolete as the 'half-world' changed.