Definition
In aviation, gear is the short form of landing gear — the wheels, struts, and supporting structure that allow an airplane to take off, land, and move on the ground. The term is used in both fixed-gear and retractable-gear airplanes, and appears in cockpit calls such as 'gear down,' 'gear up,' and 'three green' (the lights confirming the gear is down and locked).
Plain English
The wheels and the legs they're attached to underneath the airplane.
Context Anchor
You will see gear used in approach, landing, emergency, and airplane handling discussions, especially when the position of the landing gear affects how fast the airplane slows or descends.
Derivation
From Old Norse 'gervi,' meaning equipment or apparatus. The aviation use carries the same idea: 'gear' is the equipment the airplane uses to operate on the ground. The full phrase is 'landing gear,' but pilots almost always shorten it to just 'gear.'
Why Pilots Care
Gear position is one of the most critical configuration items on every flight. A retractable-gear airplane landed with the gear up causes major damage; a gear-down takeoff in a retractable airplane wastes performance and fuel. Most checklists include a gear check more than once on approach for this reason.
Intuition Check
Do not read gear here as a transmission gear, a personal item, or general equipment. In this context, gear means the airplane’s landing gear—the wheels and supporting parts used for ground movement and landing.
Example Sentence 1
On final approach, the pilot called 'gear down, three green' to confirm the landing gear was extended and locked.
Example Sentence 2
A late gear extension can increase drag and affect the aircraft's sink rate.