The Financial Information eXchange ("FIX") Protocol is an electronic "language" or set of message specifications for communicating electronic trade-related messages. It was developed through a collaboration among banks, broker-dealers, exchanges, industry utilities and associations, institutional investors, and information technology providers around the world as a common, global language for trading financial instruments electronically.
Financial trading firms use the FIX protocol to conduct fast, transparent and cost-efficient transactions. FIX is open and free. It is not software, but software developers can create commercial or open-source software around its specifications. As a leading trade-communications protocol, FIX is integral to many order management and trading systems. It supports multiple formats and types of communications between financial entities including email, texting, trade allocation, order submissions, order changes, execution reporting and advertisements. 
FIX is owned and maintained by FIX Protocol, Ltd.
The FIX Protocol was developed in 1992 for equity trading between Fidelity Investments and Salomon Brothers. FIX has become the de facto messaging standard for pre-trade and trade communication in the global equity markets, and is expanding into the post-trade space to support Straight-Through- Processing, as well as continuing to expand into foreign exchange, fixed income and derivatives markets.