Middleware is a distributed-system software that resides between applications and underlying platforms (operating systems; databases; hardware), and/or ties together distributed applications, databases or devices.
Sometimes called glue, middleware is software that provides services to applications. As the base operating system only comes with so many features, middleware's primary purpose is to facilitate communication and data exchange between software so they work together more easily.
Middleware is a software layer that connects the operating system to applications, data, and users. It provides common services and capabilities, like single-sign on (SSO) or application programming interface (API) management.