To have a proper photoelectric effect definition, we should read more about one of the fathers of physics, Albert Einstein. He was the first scientist who proposed a theory of the photoelectric effect and was even awarded the Nobel Prize in Physics for this discovery because it changed the world of science.
The photoelectric effect is a process when electrons are emitted out of material while the material is affected by electromagnetic radiation. In other words, when light falls on the metal material, electrons will be ejected from it.
Now that you know the explanation of this phenomenon, it is easier to understand what branch of physics is the best to discover it. It’s optics, which involves all the aspects of the nature of light, including interactions with materials.