What is the atomic particle that has a charge opposite to that of an electron is called?
A proton is a massive nuclear particle with a charge opposite to that of the electron. I have written before here that it would have made much more sense if the electron had been assigned a POSITIVE charge, in which case the proton would be assigned a NEGATIVE charge. As you realize, this assignment of charge is an arbitrary exercise: the point is that protons and electrons have opposite electric charges.
Such an assignment would have saved generations of quantum chemists from getting the wrong sign on their answer in quantum mechanical equations simply because they failed to count odd or even charges correctly when they counted up the electrons. It is a very easy mistake to make.