(1) A restriction, limit, or regulation imposed on a product, project, or process. (ANSI/EIA 1998)
(2) A type of requirement or design feature that cannot be traded off. (ANSI/EIA 1998)
(1) and (2) ANSI/EIA. 1998. ANSI/EIA 632|Processes for Engineering a System. Philadelphia, PA, USA: American National Standards Institute (ANSI)/Electronic Industries Association (EIA), ANSI/EIA 632-1998.