Period of time in the system life cycle during which a product is integrated into its operational environment and tested in this environment to ensure that it performs as required (adapted from (ISO/IEC 2009) Systems and software engineering vocabulary)
DAU. February 19, 2010. Defense Acquisition Guidebook (DAG). Ft. Belvoir, VA, USA: Defense Acquisition University (DAU)/U.S. Department of Defense (DoD).
Discussion as to why this is the "consensus" definition for the SEBoK.