Middleware is computer software that provides services to software applications beyond those available from the operating system. It can be described as 'software glue'. Middleware makes it easier for software developers to implement communication and input/output, so they can focus on the specific purpose of their application. It gained popularity in the 1980s as a solution to the problem of how to link newer applications to older legacy systems, although the term had been in use since 1968.

The term is most commonly used for software that enables communication and management of data in distributed applications. An IETF workshop in 2000 defined middleware as 'those services found above the transport (i.e. over TCP/IP) layer set of services but below the application environment' (i.e. below application-level APIs).