The isotopic abundance of an isotope is the fraction of the atoms in the element of that isotope. Isotopic abundancies are usually expressed in atom %.
For an element, the elemental atomic weight is the weighted average of the atomic weights of all naturally occurring isotopes of the element, weighted by the isotopic abundance for each isotope.
Fubndamentals of Nuclear Science and Engineering, J.K. Shultis, R.E. Faw, 2002.