The International Organization for Standardization (ISO) defines reference materials as follows:
Reference Material (RM)
A material or substance, one or more of whose properties are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials.
Certified Reference Material (CRM)
A reference material, accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes its traceability to an accurate realisation of the unit in which the property values are expressed by and uncertainty at a stated level of confidence.