The stakeholder that acquires or procures a product or a service from a supplier. (ISO/IEC 2010)
ISO/IEC. 2010. Systems and Software Engineering -- Life Cycle Management -- Part 2: Guide for the Application of ISO/IEC 15288 (systems life cycle processes). Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC), ISO/IEC DTR 24748-2.