Full Definition: 

The ability of systems, units or forces to provide data, information, material, and services to and accept the same from other systems, units or forces and to use the data, information, materiel and services so exchanged to enable them to operate effectively together. Information technology and national security systems interoperability includes both the technical exchange of information and the operational effectiveness of that exchanged information as required for mission accomplishment. Interoperability is more than just information exchange. It includes systems, processes, procedures, organizations, and missions over the lifecycle and must be balanced with information assurance. (CJCSI)

The ability to transfer and use information in a uniform and efficient manner across multiple organizations and information technology systems. It is the ability of two or more systems or components to exchange information and to use the information that has been exchanged. (I2F)

Definition Source: 

Chairman of the Joint Chiefs of Staff Instruction (CJCSI) 6212.01E, 21 March 2012

Information Interoperability Framework (I2F), Program Manager, Information Sharing Environment, March 2014

Australian Information Interoperability Framework, 2006

United States Code Title 44: Public Printing and Documents (2011) U.S.C. Title 44, Chap. 36, § 3601

IEEE Standard Computer Dictionary: A Compilation of IEEE Standard Computer Glossaries (New York, NY: 1990)