A group of cats is called a clowder, a glaring, or a clutter.
A clowder is the most commonly used term for a group of cats, and it is believed to have originated from the Middle English word "clod", which means "lump". A glaring is another term for a group of cats, which is derived from the Middle English word "glare", meaning "to stare". A clutter is a group of cats that live together in a colony, and it is believed to have originated from the Old English word "cloter", meaning "to huddle".