Atomic (proton) number is the number of protons in the nucleus of an atom.
Mass (nucleon) number is the total number of protons and neutrons in the nucleus of an atom.
An isotope is an atom of the same element, i.e. it has the same number of protons/same atomic number, but has a different number of neutrons/different mass number. Two atoms with the same atomic number but different mass numbers are isotopes
see 7.02 for example