Middleware in the context of distributed applications is software that provides services beyond those provided by the operating system to enable the various components of a distributed system to communicate and manage data. Middleware supports and simplifies complex distributed applications. It includes web servers, application servers, messaging and similar tools that support application development and delivery. Middleware is especially integral to modern information technology based on XML, SOAP, Web services, and service-oriented architecture.

Middleware often enables interoperability between applications that run on different operating systems, by supplying services so the application can exchange data in a standards-based way. Middleware sits "in the middle" between application software that may be working on different operating systems. It is similar to the middle layer of a three-tier single system architecture, except that it is stretched across multiple systems or applications. Examples include EAI software, telecommunications software, transaction monitors, and messaging-and-queueing software.

IBM, Red Hat, Oracle Corporation and Microsoft are major vendors providing middleware software. Vendors such as Axway, SAP, TIBCO, Informatica, Pervasive and webMethods were specifically founded to provide Web-oriented middleware tools. Groups such as the Apache Software Foundation, OpenSAF and the ObjectWeb Consortium (now OW2) encourage the development of open source middleware. Microsoft .NET “Framework” architecture is essentially “Middleware” with typical middleware functions distributed between the various products, with most inter-computer interaction by industry standards, open APIs or RAND software licence.