the social class between the aristocracy or very wealthy and the lower working class: people in business and the professions, highly skilled workers, well-to-do farmers, etc. are now generally included in the middle class
of, having to do with, or characteristic of the middle class: often used variously to connote conventional, smug, materialistic, etc. attitudes
See middle class in American Heritage Dictionary 4
The socioeconomic class between the working class and the upper class, usually including professionals, highly skilled laborers, and lower and middle management.