Plain English Meaning
The switch in the cockpit the pilot moves up or down to raise or lower the landing gear.
Full Definition
The cockpit control used by the pilot to command the landing gear to extend (down) or retract (up). It is typically a two-position switch or lever, often shaped like a small wheel, located on the instrument panel within easy reach of the pilot.
Context Anchor
Seen in airplanes with retractable landing gear, especially during after-takeoff and before-landing checklist steps.
Derivation
Gear' is shorthand for 'landing gear,' the wheels and supporting structure the airplane lands on. 'Selector' comes from Latin selectus, meaning 'chosen' — the switch lets the pilot between gear up and gear down.