CRT is short for cathode ray tube. That is the old, heavy glass screen that televisions and computer monitors used before flat screens. Deep in the back of the box, a gun fires a thin beam of electrons (tiny bits of electricity) at the front glass, which has a special coating; where the beam hits, a dot of light appears, and many dots make the picture. The gun has to sit a long way from the glass, so the whole machine is deep, fat, and hard to lift — that is why old TVs take up so much room. People still say CRT when they mean that old kind of screen, or when engineers talk about how the tube works. You will not hear it for a phone or a thin modern TV. Though actually, some game players still look for CRT screens, because old game machines were made for that picture and can look odd on a new one.