ECN is an abbreviation of Electronic Communication Network. The phrase ECN is by no means exclusive to the financial markets and the online trading industry, but we use it too. In Forex, ECN trading means electronically sending (communicating) orders to a venue (network).
Consider a stock exchange; it’s a venue where participants can use online (electronic) platforms to submit orders, and they are matched against other participants. In the same way that you can’t just open an account with the New York Stock Exchange, so you need a broker to give you access, an ECN forex broker provides access to a decentralised foreign exchange environment.