Probably more accurately called a
pizza table, but
pizza hat seemed to be easier to say.
This is a piece of plastic usually with three legs and a flat disc top or three spokes intersecting at the center that is used in a pizza box to prevent the center of the box from crushing down, touching the hot, gooey cheese in the center of the pizza, and causing it to pull up when the box is opened, ruining the aesthetic beauty of the pizza, as well as making a mess.
Also makes the perfect cat toy--even better if you leave some of the cheese on it.