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Traders101.com reproduces this definition here under license for your convenience.

ECN or "electronic communications networks" are computerized order matching systems. When a trading order to buy or sell a particular security is sent to an ECN, the order is matched with an equal and opposite order in a second or less. Until recently, only big institutions could trade using ECNs because of their capital costs.This allowed them to buy and sell stocks outside of the regular market hours (when a lot of earnings announcements, and other important news announcements took place). This was fairly bad news for small investosr, who of course had to wait for the actual market to open. This all changed in 97, when Island opened to the public. The growth in trading via ECNs has been fast, taking business away from more traditional methods. Due to execution speed and transparency, ECNs have become the weapon of choice for day trading, although their use in other forms of trading may be limited.