A group of cats is commonly referred to as a 'clowder' or a 'clutter'.
A clowder of cats is a term that has been used for centuries to refer to a group of cats. The origin of the term is uncertain, but it is thought to be derived from the Middle English word 'clodder', which means 'a mass or lump'. A clutter of cats is another term that is used to refer to a group of cats, but it is not as widely used.