Two atoms are said to be isotopes if

Isotopes are defined as atoms of the same chemical element that possess the same number of protons but a different number of neutrons [2]. Since the number of protons determines the atomic number (Z), all isotopes of an element share the same atomic number [2]. However, the mass number (A) is the sum of protons and neutrons in the nucleus [2]. Because isotopes have varying numbers of neutrons, they consequently have different mass numbers [2]. For instance, carbon naturally occurs as three isotopes: carbon-12, carbon-13, and carbon-14, all having six protons but different neutron counts. In contrast, atoms with the same mass number but different atomic numbers are called isobars, and those with the same number of neutrons but different atomic numbers are called isotones.

Sources

1. [1] https://gml.noaa.gov/ccgg/isotopes/chemistry.html
2. [2] https://www.energy.gov/science/doe-explainsisotopes