1. The ability of systems, units, or forces to provide services to and accept[?] services from other systems, units or forces and to use the services so exchanged to enable them to operate effectively together.
2. The condition achieved among communications-electronics systems or items of communications-electronics equipment when information or services can be exchanged directly and satisfactorily between them and/or their users. The degree of interoperability should be defined when referring to specific cases.
Source: from Federal Standard 1037C and from MIL-STD-188 and from the Department of Defense Dictionary of Military and Associated Terms
In the UK there is a egovernment-based interoperability initative called e-gif.
wikipedia.org dumped 2003-03-17 with terodump