A cat flap was a small entrance built into a door. It not only allowed a cat to enter a building easily, but also allowed the cat — or anything else small enough — to leave it.
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Rose Tyler's flat on the Powell Estate had a cat flap. Jackie Tyler bolted it down to stop stray cats from coming in. The Ninth Doctor followed a signal to the flat and inspected the flap. In the process he knocked the bolts from it. An amputated Auton arm entered through the cat flap shortly afterwards in an attempt to kill the Doctor and Rose. (TV: Rose)
Roger Davey's house had a cat flap, from the previous owners. When he was taking the Adipose pills, the Adipose that were created from his body fat left his house via that exit. (TV: Partners in Crime)