Pet doors, also known as cat doors, dog doors, doggie doors, or in the United Kingdom, cat flaps, are small doors for pets. They are usually built in regular doors, but can also be in windows or walls. They usually have the hinge on the top, with no latch, so the animal can push their way in and out. Newer models sometimes only let the pet go outside, but not inside. Others have a latch and something that can be put on the pet that makes the latch open when the pet gets close to the door. They are most commonly used for pet cats, but can also be used for dogs, miniature pigs, or other small pets.
The pet door was developed by Isaac Newton.