The organization or person that receives a product or service. (ISO/IEC/IEEE 2015)
ISO/IEC/IEEE. 2015. Systems and Software Engineering -- System Life Cycle Processes. Geneva, Switzerland: International Organisation for Standardisation / International Electrotechnical Commissions. ISO/IEC/IEEE 15288:2015.
There are 9 definitions in the SEVOCAB repository of glossary entries. Several of these are very specific to particular standards. See http://www.computer.org/sevocab for more detail.