Demimonde (French for "half-world") is a polite 19th century term that was often used the same way we use the term "mistress" today. In the 19th century it primarily referred to a class of women on the fringes of respectable society supported by wealthy lovers (usually each had several). The term is also used to refer to these women as a group, and the social circles they moved in. As a group, the demimonde did not form a 'society' any more than modern prostitutes form a society. However, they did represent a specific class of women in the latter half of the 19th century and into the early 20th century who, though not respectable, were commonplace fixtures in the upper class of French, English and, to some extent, American society. In the United States and Britain, they were (and still are) also often referred to as courtesans, though the term courtesan in the 19th century applied to a profession (as the term "prostitute" describes a profession), whereas Demimonde/Demimondaine was
Source:
http://en.wikipedia.org/wiki/Demimonde