Longman Dictionary of Contemporary English
cat flap
noun
EXAMPLES FROM CORPUS
▪ Became confident and prised the cat flap open again.
▪ I hear the thump of the cat flap as Cat O'Fun tumbles through it without the feline grace of his fellows.
▪ It's only a matter of time till he learns to use the cat flap.
▪ Somebody had left me a note, though, through the cat flap in my flat door.
Wiktionary
cat flap
alt. Small hinged panel, usually cut into a door, with an opening just big enough for a cat to enter. n. Small hinged panel, usually cut into a door, with an opening just big enough for a cat to enter.