Final answer:
The weighted average of the masses of naturally occurring isotopes for an element is called atomic mass.
Step-by-step explanation:
The weighted average of the masses of naturally occurring isotopes for an element, relative to carbon-12, is called the atomic mass of the element.
The atomic mass of an element is calculated by multiplying the mass of each isotope by its fractional abundance and summing up the products.
For example, the atomic mass of carbon is calculated by multiplying the mass of the carbon-12 isotope by its fractional abundance (98.89%) and the mass of the carbon-13 isotope by its fractional abundance (1.11%).