The undercarriage is the whole set of wheels under a plane, together with the legs, struts and springs that hold those wheels and take the shock when the plane lands. It is not just the tyres — it is the entire assembly that folds up into the body of the plane after take-off and comes down again before landing. This is why you hear pilots and news reports say things like 'the undercarriage failed to come down' or 'the plane landed with its undercarriage up'. In American English people say 'landing gear' far more often, and 'landing gear' is now common everywhere, including among British pilots; 'undercarriage' feels a bit more British and a bit older, though it is still completely normal in British writing and in aviation. The word also gets used for the frame and wheels under a train carriage, a lorry, a caravan, or an old horse-drawn cart — anything where a body sits on top of a structure with wheels. Mechanics sometimes use it loosely for 'everything underneath the vehicle', which is why a garage may say your car's undercarriage is rusty when they really mean the whole underside. And people joke with it about a body — someone's legs, or the underside of an animal — but that is playful, not a real meaning, and students should not use it that way in writing.