A group of kittens is called a kindle. The word "kindle" comes from the Middle English word "kindel," which means "offspring." It is derived from the term "kindelen," which means "to give birth to."
A group of kittens can also be referred to as a litter, which is primarily used to describe the offspring of mammals, specifically the offspring of one birth from the same mother. Another term used to described a group of kittens is an intrigue.
Almost all animals have at least one unique collective noun used to refer to a group of them at once. A group of cats is called a clowder, a clutter, a glaring or a pounce.