a combat support agency in the Department of Defense responsible for developing and operating and supporting information systems to serve the needs of the President and the Secretary of Defense and the Joint Chiefs of Staff
From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
(DISA) Formerly called the Defense Communications Agency (DCA), this is the government agency responsible for managing the Defense Data Network (DDN) portion of the Internet, including the MILNET. Currently, DISA administers the DDN, and supports the user assistance services of the DDN NIC.