Statement that identifies a product* or process operational, functional, or design characteristic or constraint, which is unambiguous, testable or measurable, and necessary for product or process acceptability. (ISO/IEC 2007)
ISO/IEC. 2007. Systems and Software Engineering -- Recommended Practice for Architectural Description of Software-Intensive Systems. Geneva, Switzerland: International Organization for Standards (ISO)/International Electrotechnical Commission (IEC), ISO/IEC 42010:2007.
A requirement is “a statement that identifies a system, product or process characteristic or constraint, which is unambiguous, clear, unique, consistent, stand‐alone (not grouped), and verifiable, and is deemed necessary for stakeholder acceptability.” (INCOSE 2010)
INCOSE. 2010. Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities. Version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1: 362.