Why is a H+ ion referred to as a proton?

An atom of hydrogen contains 1 proton, 1 electron and 0 neutrons. When hydrogen loses an electron to become H+ only a proton remains. 

We can work out the number of neutrons an atom has by deducting the atomic number from the mass number. We know that hydrogen has 0 neutrons because 1 - 1 = 0.