In analytical chemistry, a trace element is an element in a sample that has an average concentration of less than 100 parts per million measured in atomic count, or less than 100 micrograms per gram.
In biochemistry, a trace element is a dietary mineral that is needed in very minute quantities for the proper growth, development, and physiology of the organism.[1]
In geochemistry, a trace element is a chemical element whose concentration is less than 1000 ppm or 0.1% of a rock's composition. The term is used mainly in igneous petrology. Trace elements will either prefer liquid or solid phase. If compatible with a mineral, it will prefer a solid phase (E.G. Ni compatible with Olivine). If it is incompatible with an element it will prefer a liquid phase. The measurement of this ratio is known as the partition coefficient. Trace elements can be substitutes for network-forming cations in mineral structures. Minerals do not have to contain trace elements I.e they do not have to appear in in the mineral's chemical formula.