Why we call protium as isotope of hydrogen despite they have no neutron in thier nucleus?

it is known that the isotopes of an element is determined by its variation in neutron numbers but in the case of hydrogen which has 3 isotopes that is protium, deutrium,tritium last two has 1 and 2 neutrons respectly but protium has no neutron then why we called them as isotope of hydrogen besides they have no neutron with them.