An important part of the plane
that supports the whole weight of the plane, as well as a rough and bouncy landing. They are wheels and struts that connect to the underside of a plane, and usually retract while the airplane is airborne.
These have to be specially designed because early tires on the gear built up a substantial amount of static electricity. Bad for the plane and the electronics, as well as that first person getting off the plane.
In some rough landings, the tires have locked up and caught fire. Luckily it was at the beginning or end of the flight, and the people get to use those cool evacuation slides.
One of the more amusing stories I've heard involving military fighter jets was when a pilot became curious and wanted to see what would happen if the landing gear was retracted while the plane was sitting on the runway. He pulled the lever, and *Crunch* caused millions of dollars of damage to the wings and fuselage. Now, special safeguards are in place on that model preventing the wheels from being retracted unless the plane is off the ground.
Also, the legs a semi-trailer stands on when not attached to a tractor are also called landing gear.