FIX (Financial Information eXchange protocol) is a protocol that is used by financial participants to exchange market data and trading information.
In most common parlance, ‘FIX’ refers to the TCP-based, tagvalue protocol which remains one of the most common ways for financial market participants to exchange order messages.
However increasingly FIX now refers to a family of protocols. These include the TCP-based protocol but now also include other protocols that are more efficient for other financial market use cases, notably FIX SBE.