ECN
An electronic communications network is an electronic trading system that automatically matches buy and sell orders at specified prices. Subscribers to ECNs such as institutional investors, brokers and market makers trade directly with an ECN. In the United States. ECNs are required to register with the SEC. Individual investors can only trade on an ECN via a broker who is a subscriber. ECN subscribers usually use limit orders to make their trades. The order is posted on the ECN for other subscribers to view. If a matching buy or sell order is not available it won't be executed until one comes in. Alternatively, the ECN may send the order to another ECN, to an exchange or to a market maker to be matched.
See also: Dark Pools, Algorithmic Trading, Flash Orders, Co-Location