The undercarriage or landing gear is the part of an aircraft on which it lands: usually wheels, but sometimes skis or floats (for landing on snow or water respectively), plus their supporting structure, often including some form of shock absorber.
Many aircraft have an undercarriage which retracts, to improve streamlining while in flight. Often, it is then hidden behind undercarriage doors.
Modern aircraft typically have three wheels, or sets of wheels, touching the ground when at rest, giving a tripod effect and increasing stability. Those with a wheel or wheels beneath the nose (a nose wheel) have what is called tricycle undercarriage with the leading wheel(s) steering the aircraft when taxying (e.g. the Boeing 747). Those with a wheel or wheels (or skid) at the rear (a tail wheel) are commonly called taildraggers (e.g. the Hawker Hurricane). Aircraft rarely have both a nose wheel and a tail wheel but sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case the tail strikes the ground during take-off. Concorde, for instance, had a retractable tail "bumper" wheel.
Some planes use wheels only for take off and drop them afterwards to gain the improved streamlining without the complexity, weight and space requirements of a retraction mechanism. In this case, landing is achieved on skids or similar simple devices. Examples include the Messerschmitt Me 163 Komet and the Messerschmitt Me 321 Gigant.
Undercarriage is also a euphemism for human genitals.