The Financial Information eXchange (FIX) protocol was developed to facilitate real-time financial transactions.
FIX is primarily used by banks, brokerage firms, information technology providers, exchanges, and institutional investors, to electronically communicate trade-related messages. In its simplest form, an application generates FIX messages which are then transmitted using a FIX Engine. A second FIX engine then receives the messages, and re-transmits them to its respective application.
Example
The stock exchange application generates messages and transmits them using the stock exchange's FIX engine. In this example, the stock exchange's FIX engine is the "initiator".
The trading firm's FIX engine then receives the messages, and one or more of the firm's applications accepts the messages in FIX format. The trading firm's FIX engine is called the "acceptor".