Any element having an average concentration of less than about 100 parts per million atoms (\(\text{ppma}\)) or less than \(100\ \frac{\unicode[Times]{x3BC}\text{g}}{\text{g}}\).
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 [Terms] [Paper]