The term "middle class" is first attested in James Bradshaw's 1745 pamphlet Scheme to prevent running Irish Wools to France. Another phrase used in Early modern Europe was "the middling sort".
The term "middle class" has had several, 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. In France, the middle classes helped drive the French Revolution. Another definition equated the middle class to the original meaning of capitalist: someone with so much capital that they could rival nobles. In fact, to be a capital-owning millionaire was the essential criterion of the middle class in the industrial revolution.
The modern usage of the term "middle-class", however, dates to the 1913 UK Registrar-General's report, in which the statistician T.H.C. Stevenson identified the middle class as that falling between the upper-class and the working-class. Included as belonging to the middle-class are: professionals, managers, and senior civil servants. The chief defining characteristic of membership in the middle-class is possession of significant human capital.
Within capitalism, "middle-class" initially referred to the bourgeoisie and the petite bourgeoisie. However, with the impoverisation and proletarianisation of much of the petit bourgeois world, and the growth of finance capitalism, "middle class" came to refer to the combination of the labour aristocracy, the professionals, and the white collar workers.
The size of the middle class depends on how it is defined, whether by education, wealth, environment of upbringing, social network, manners or values, etc. These are all related, but are far from deterministically dependent. The following factors are often ascribed in modern usage to a "middle class":