A cat flap, also known as a pet door, is a small door that allows pets, especially cats, to come and go as they please without the need for human intervention. Cat flaps usually have a hinged or sliding panel that swings open when a pet pushes on it with their body weight, allowing the pet to pass through the opening. Cat flaps can be installed in various types of materials, including wood, plastic, and metal, and are commonly used in homes to allow pets to have easy access to the outdoors.