What is meant by "Middle Market" ("MM")?

"Middle Market" can refer to Investment Banks that specialize in "middle market" deals or are hired by "middle market" companies ie, not your household, huge Fortune 500 companies, but the companies that make up the majority of the world's economy. The Middle Market is huge and the phrase "middle market" is often used to get across the point that the firm, Investment Bank or deal is not from a Bulge Bracket. Sometimes the companies that do middle market deals are called "Boutique" Banks or "middle market" banks.