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)
ISO/IEC. 2009. Systems and Software Engineering Vocabulary (SEVocab) - ISO/IEC 24765. in International Organization for Standardization (ISO)/International Electronical Commission (IEC) [database online]. Geneva, Switzerland, 2009 [cited December 21 2009]. Available from http://pascal.computer.org/sev_display/index.action.
Discussion as to why this is the "consensus" definition for the SEBoK.