Question

What is the mass of an atom's nucleus called?

Answer 1

This is a new one on me. I don't know that there is a name for the mass of an atom's nucleus. Mass of nucleons would be appropriate but I've not heard that applied.

The name for the NUMBER of protons and neutrons (nucleons) is the mass number and almost all of the mass is centered in the nucleus. The mass of the whole ATOM is the atomic mass of the atom but that includes the mass of the electrons, too.