There is a computer network, similar to Internet in architecture but not connected to actual Internet, that is used by the military. What is the term to describe such network?
A private network that is accessible only to the employees of an organisation is called an intranet.
a computer network with restricted access, as within a company, that uses software and protocols developed for the Internet.
The same definition would apply in the military context. The difference might be that an elevated level of security precautions is imposed on such a network.
Wide area Network or WAN.
A wide area network (WAN) is a telecommunications network or computer network that extends over a large geographical distance. Wide area networks are often established with leased telecommunication circuits.
Business, education and government entities use wide area networks to relay data among staff, students, clients, buyers, and suppliers from various geographical locations. In essence, this mode of telecommunication allows a business to effectively carry out its daily function regardless of location. The Internet may be considered a WAN.