The average mass of all the isotopes of an element is known as its atomic mass. More specifically, the atomic mass of an element is the weighted average of mass of all the naturally occurring isotopes. Thus, given the relative abundance of each isotope of an element and their masses, we can calculate the atomic mass of that particular element. For example, uranium has 3 naturally occurring isotopes: U-234, U-235 and U-238. These isotopes have relative abundances of (approximately) 0.005%, 0.72% and 99.275%, respectively. Thus, the atomic mass of uranium is calculated as:
234 x 0.005/100 + 235 x 0.72/100 + 238 x 99.275/100
= 237.9782 u.
We can, similarly, find the average atomic mass of any element as long as we are given the relative abundances of each of its naturally occurring isotopes.
Hope this helps.