In general usage, this term now denotes any large self-service retail outlet. The emphasis is on ease of selection, persuasive advertising, high turnover and equally high profits. Costs are kept at a minimum by economizing on staff, customers serving themselves with little or no supervision, bulk-buying to match the high turnover and often, due to the location of the premises away from town centres, low rents and rates.
Originally, a supermarket described a collection of traders, each operating on their own account but sharing common facilities under one roof. When this meaning was abandoned for the more general usage described above, theoretical distinctions were still made according to the size of the business. Those of less than 186 square metres in floor space were strictly ‘superettes’, those with a larger area were super¬ markets proper and those which were very large and carried vast quantities of goods (often specializing in either durables or household furniture) were to be known as hypermarkets. Those distinctions have become blurred in usage.
Reference: The Penguin Business Dictionary , 3rd edt.