To acquire enough shares of a particular security type, such as those of a firm in a niche industry, or to hold a significant commodity position to be able to manipulate its price. An investor needs deep pockets to be able to corner a market.
It can also mean to accumulate a major share of economic activity in a particular area. A phone company that dominated 90% of the wireless market could be said to have cornered the market.
The Securities and Exchange Commission and Commodity Futures Trading Commission regulate and monitor the securities and commodities markets, and attempt to prevent and prosecute illegal trading behavior. Large institutions can often corner a market through legal means. A company that has cornered the market has a significant competitive advantage. However, any time a company has a large market share, it may be scrutinized by the Department of Justice's Antitrust Division - especially if competitors complain. Indeed, Microsoft faced such a fate because of its large share of the computer operating system market.