Each isotope has a special name derived from latin (protium, deuterium, and tritium). what structural feature do these names refer to in the atom?

An isotope of any element is the same, with a variation in the neutrons of the nucleus.

The mass number change but the atomic number doesn't.

In this case, protium, deuterium, and tritium are all hydrogen isotopes.

Protium is 1H or Hydrogen-1 is without neutrons.

Deuterium is 2H  or Hydrogen-2 has one neutron.

Tritium is 3H or Hydrogen-3 has two neutrons.