Definition of: isotope

(ī′sə·tōp) noun Physics
Any of two or more forms of an element having the same atomic number and similar chemical properties but differing in atomic weight and radioactive behavior. The accepted atomic weight of an element is the average of the nuclear masses of all the isotopes it may contain. [<ISO- + Gk. topos place]
—i·so·top·ic (ī′sə·top′ik, -tō′pik) adjective
—i·sot·o·py (ī·sot′ə·pē) noun