An alternate form of an element that has the usual number of protons but a nonstandard number of neutrons; the fewer or additional neutrons give the isotope a different atomic weight than the regular element and may make the isotope radioactive, but otherwise an isotope has the same chemical action as the regular element. Because of this, isotopes (such as radioactive carbon) are used as tracers in biological systems or processes.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.