Complex sentence: A mononuclidic element is a chemical element which is found essentially as a single nuclide, of only one atomic mass. Thus, the element's natural isotopic abundance is dominated either by one stable isotope or by one very long-lived isotope.
Split into simpler sentences: A mononuclidic element is a chemical element which is found essentially as a single nuclide (which may or may not be stable). This single nuclide will have a characteristic atomic mass. Thus, the element's natural isotopic abundance is dominated either by one stable isotope or by one very long-lived isotope.