The chemical, physical, or biological quality or condition exhibited at either a single site or an aggregation of sites that are representative of the least-impacted and attainable condition. Reference conditions are used to describe reference sites.

(Measurement and Inspection) The performance specifications for measuring sensors are typically given for reference conditions. Reference conditions are usually 20°C and 1 atmosphere of pressure. In addition, a reference target must also be described in the specifications. For laser measuring devices, a white ceramic target is often used. For ultrasonics, a square metal target is typically called out.