FIX-3-5.5 is a messaging specification within the Financial Information eXchange (FIX) protocol. It defines a language used for financial transactions between parties, such as financial institutions, brokerages, and market makers. The language consists of message types for order entry, order cancellation, order status, and other trading-related functions. There are various versions of FIX, but FIX-3-5.5 is the most commonly used in the United States. This version defines the messages, tags, and data elements that are used in the messages.