In the computer industry, middleware is a general term for 
any programming that serves to "glue together" or mediate 
between two separate and often already existing programs. A 
common application of middleware is to allow programs 
written for access to a particular database to access other 
databases. The systematic tying together of disparate 
applications, often through the use of middleware, is known 
as enterprise application integration.
Or,
Software that mediates between an applications program and 
a network. It manages the interaction between disparate 
applications across the heterogeneous computing platforms. 
The Object Request Broker (ORB), software that manages 
communication between objects, is an example of a 
middleware program.

Middleware is computer software that connects software 
components or applications. The software consists of a set 
of enabling services that allow multiple processes running 
on one or more machines to interact across a network. This 
technology evolved to provide for interoperability in 
support of the move to coherent distributed architectures, 
which are used most often to support and simplify complex, 
distributed applications. It includes web servers, 
application servers, 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.

It sits "in the middle" between application software 
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 database systems, 
telecommunications software, transaction monitors, and 
messaging-and-queueing software.

The distinction between operating system and middleware 
functionality is, to some extent, arbitrary. While core 
kernel functionality can only be provided by the operating 
system itself, some functionality previously provided by 
separately sold middleware is now integrated in operating 
systems. A typical example is the TCP/IP stack for 
telecommunications, nowadays included in virtually every 
operating system.

In simulation technology, middleware is generally used in 
the context of the high level architecture (HLA) that 
applies to many distributed simulations. It is a layer of 
software that lies between the application code and the run-
time infrastructure. Middleware generally consists of a 
library of functions, and enables a number of applications –
 simulations or federates in HLA terminology – to page 
these functions from the common library rather than re-
create them for each application