Dark Pools
Dark pools, or dark liquidity pools, are private electronic trading venues that operate separately from public exchanges. They don't publish bid and offer prices and don't promptly publish transaction prices. They enable institutional investors and traders to buy and sell large blocks of securities anonymously, and also more cheaply than on public exchanges. This protects the privacy of an investment and reduces market movements caused by copycat trading. Dark pools are widespread in the United States where they accounted for more than 10 percent of trading as of 2009. They have to be registered with the SEC. The are becoming more common in Europe following the introduction in 2007 of MIFID, the EU's Markets in Financial Instruments Directive that increased competition in share trading.
See also: ECN, Flash Orders, Co-Location, Algorithmic Trading