The term Market Sector is used to describe a set of businesses that are buying and selling such similar goods and services that they are in direct competition with each other. Analysts divide the stock market itself into market sectors so that shares of companies that are in direct competition are listed alongside each other.
In the bond market it refers to the division of the market by the type of issuer. E.g. government, state, corporate, or utility.
Markets are split into sectors using the Global Industry Classification Standard.