A. A social class of business or professional people, also known as the Bourgeoisie.

B. Social class broadly defined occupationally as those working in white-collar and lower managerial occupations; sometimes defined by reference to income levels or subjective identification of the participants in the study.

C. the social class between the lower and upper classes (my favorite)

D. the long version from Wiki:

The middle class in this article refers to people neither at the top nor at the bottom of a social hierarchy. The term "middle class" has a long history and has had many, sometimes contradictory, meanings. It was once defined by exception as an intermediate social class between the nobility and the peasantry of Europe. While the nobility owned the countryside, and the peasantry worked the countryside, a new bourgeoisie (literally "town-dwellers") arose around mercantile functions in the city. This had the result that the middle class were often the wealthiest stratum of society (whereas today many take the term to refer by definition to the only-moderately wealthy.)

Descending from this distinction, the phrase "middle class" came to be used in the United Kingdom during the Industrial Revolution to describe the professional and business class, as distinct from both the titled nobility and the landed gentry on the one hand and the agricultural and (increasingly) industrial laborers on the other.

Throughout the twentieth century, the titled nobility of the United Kingdom became less homogeneous. This was because of the increasingly eclectic background of new creations, most of which were politically driven by the so-called middle class, and the declining power of the House of Lords relative to the House of Commons after the Parliament Act 1911. So far as the hereditary element of class was concerned, the titled upper class became less numerous because of the near cessation of new hereditary creations after the Life Peerages Act 1958. This was coupled with the natural rate of extinction of existing hereditary titles and the near abolition of the hereditary element of the House of Lords at the end of the twentieth century. At this point, hereditary titles are in no way the key to being "upper class," although they do lend a distinctive panache within the upper class. By the beginning of the twenty-first century, Tim Hames was able to write in The Times (on 9 October 2006): "In a world in which David Cameron, whose mother-in-law is Lady Astor, and George Osborne, the son of a baronet, can claim to be “upper middle class” and that claim passes without much challenge, the upper class has plainly been reduced to such a rump that it now consists exclusively of the Royal Family."

So, like anything, it is all in your point of view.