A tetrad is a group of four things. If you've ordered four pizzas for dinner, you can announce that you are serving a tetrad of pizzas.
You can use the noun tetrad to talk about a set of four numbers in math, or a group of four things in various branches of science — one kind of tetrad is a group of four spores in one specific type of yeast. It's more common in non-scientific speech to say quartet or quadruplet. Tetrad comes from a Greek root, the word tetras, "group of four," from tetra, or "four."