Middle class, not middle income.
The middle class is any class of people in the middle of a societal hierarchy. In Weberian socio-economic terms, the middle class is the broad group of people in contemporary society who fall socio-economically between the working class and upper class.
The common measures of what constitutes middle class vary significantly between cultures. In urban India, for example, a family is considered middle class if it resides in an owner-occupied property. In the United States and Canada many families where the primary income-earner is employed in a white collar job are considered part of the middle class. Moreover, most North Americans would take issue with a definition of middle-class which excluded the working class, i.e. 'classic Weberian'. (Hard work is generally held in high honour, fairness and equality are common law, and the North American economy was built upon traditionally labour intensive industries.)
Middle class - Wikipedia, the free encyclopedia