An electronic communications network (ECN) is a trading venue that is not regulated as a stock exchange, usually a matched bargain ATS that offers lower costs than the primary exchanges. It is also common for an ECN to offer different terms to a stock exchange, for example requiring less disclosure or orders (e.g. being a dark pool) or operating longer trading hours.
Inter-dealer brokers also a way of trading outside an exchange.
ECNs have increased their market share and are serious competitive threats to stock exchanges. Some have become exchanges, with more likely to follow. The differences between an ECN and an exchange are regulatory, so this is almost purely a matter of regulatory approval.