Wiktionary
n. An opening in a door for pets to come in and out of the house without requiring a human to open the door.
Wikipedia
A pet door or pet flap (also referred to in more specific terms, such as cat flap, cat door, dog door, or doggie door) is a small portal in a wall, window or human door to allow pets to enter and exit a house (or other structure) on their own without needing a person to open the door. Originally simple holes, the modern form is a hinged and often spring-loaded panel or flexible flap, and some are electronically controlled. They offer a degree of protection against wind, rain, and larger-bodied intruders entering the dwelling. Similar hatches can let dogs through fences at stiles. A related concept is the pet gate, which is easy for humans to open but acts as a secure pet barrier, as well as innovative automated left- or right-handed pet doors.
Usage examples of "pet door".
Mine would have shown up on the inner and outer aspects of the pet door, on the edge of the desk, on the back doorknob, in other places so numerous I could hardly recall.
He sprinted down the hall, disappeared into the kitchen, where the pet door gave him access to the rear yard.
The dog plucked it from Sam's hand, held it in his teeth, and went back into the house through the pet door.
If it was closer to the ground I'd say it was a pet door, but the only pet who could jump through it at that height would be a kangaroo, and it's too small for kangaroos.