The force that holds protons and neutrons together in the nucleus of an atom is called the strong nuclear force.

Here's why it's called that:

* Strong: It's the strongest fundamental force known in the universe, even stronger than the electromagnetic force that repels protons due to their positive charge.

* Nuclear: It operates within the nucleus of an atom, holding the protons and neutrons together.

Let me know if you'd like to learn more about the strong nuclear force!