The DIESIS project  gives the following definition:
HLA (High Level Architecture) is an IEEE standard developed by the United States department of defence . It allows the exchange of information among several and
heterogeneous simulators during simulation running. Communications are possible thanks to the RTI (Run Time Interface), which provides a set of services useful for simulations to coordinate their actions and the information exchange. To communicate through the RTI, each simulator must be RTI compliant:
- By implementing/using a RTI compliant API.
- By respecting the OMT (Object Model Template) for information (objects, events, etc.) exchange and for specific simulator modelling.
- By respecting a number of rules concerning communications between simulations and concerning each simulation. The HLA ability can be useful to exchange information between electricity and telecom simulators.
- ↑ http://www.diesis-project.eu/
- ↑ 1516.3:2003 IEEE Recommended Practice for High Level Architecture (HLA) Federation Development and Execution Process (FEDEP), 2003.