cat flap 
(also cat door)
noun
A small hinged flap in an outer door, through which a cat may enter or leave a building.He keeps throwing glances at the cat flap in the back door but it will be a couple more weeks before we let him venture outside....
- He likes to sit on the windowsill, and to look out of the French doors and through the cat flap.
- Within the six weeks he grew so fast that he could no longer walk in and out of the house through the cat flap on the front door.