Any element having an average concentration of less than about 100 parts per million atoms (ppma) or less than 100 µg/g.
Source:
PAC, 1979, 51, 2243. 'General aspects of trace analytical methods—IV. Recommendations for nomenclature, standard procedures and reporting of experimental data for surface analysis techniques' on page 2246 (https://doi.org/10.1351/pac197951112243)