The degree to which a property or a constituent is uniformly distributed throughout a quantity of material. A material may be homogeneous with respect to one analyte or property but heterogeneous with respect to another. The degree of heterogeneity (the opposite of homogeneity) is the determining factor of sampling error.
PAC, 1990, 62, 1193. 'Nomenclature for sampling in analytical chemistry (Recommendations 1990)' on page 1201 (https://doi.org/10.1351/pac199062061193)