A device, computer program, or system that behaves or operates like a given system when provided a set of controlled inputs. (ISO/IEC 2010)
ISO/IEC/IEEE. 2010. Systems and Software Engineering - System and Software Engineering Vocabulary. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC)/ Institute of Electrical and Electronics Engineers (IEEE). ISO/IEC/IEEE 24765:2010.