Atomic mass can be used to describe both an element as a whole and a specific isotope of that element.
The atomic mass of an element is the weighted average of the masses of all its naturally occurring isotopes, taking into account their abundance. Since isotopes of an element have different numbers of neutrons, they have slightly different atomic masses. Therefore, when referring to the atomic mass of an element, it represents the average mass of all the isotopes of that element found in nature.
On the other hand, the atomic mass of a specific isotope is the mass of that particular isotope. Isotopes of an element have different atomic masses because the number of neutrons differs, while the number of protons remains the same (as it determines the element). By specifying a particular isotope, you are referring to its unique atomic mass.
For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. The atomic mass of carbon is the weighted average of the masses of these isotopes, where carbon-12 is the most abundant. However, if you refer to the atomic mass of carbon-14, you are specifically mentioning the mass of that isotope, which is approximately 14 atomic mass units (amu).