In its most general sense, middleware is computer software that provides services to software applications beyond those available from the operating system. Middleware can be described as "software glue". Thus middleware is not obviously part of an operating system, not a database management system, and neither is it part of one software application. Middleware makes it easier for software developers to perform communication and input/output, so they can focus on the specific purpose of their application.
Read more about Middleware: Middleware in Distributed Applications, Other Examples of Middleware, Boundaries, Origins