marker

1) Small amount of an easily detected substance that can be used to follow and quantify the flow of materials or movement of organisms not otherwise visible or detectable by ordinary means. 2) An isotope of an element, a small amount of which may be incorporated into a sample of material in order to follow the course of that element through a chemical, biological, or physical process, and thus also follow the larger sample. The tracer may be radioactive, in which case observations are made by measuring the radioactivity. (Source: ECHO1)