Plain English Meaning
The leg between the wheel and the airplane that holds the wheel in place and cushions the bumps when you land or taxi.
Full Definition
A structural leg of the landing gear that connects the wheel assembly to the airframe and absorbs the loads of landing, taxi, and takeoff. On most airplanes the strut also acts as a shock absorber, using either compressed air and oil (an oleo strut), a spring steel leg, or rubber/bungee cushioning to soften touchdown forces.
Context Anchor
Seen during preflight inspection when checking the landing gear for proper extension, leaks, damage, or an uneven stance.
Derivation
Strut' comes from older Germanic roots meaning a rigid support that holds something up under load. That is exactly its job here: a strong leg that holds the airplane's the and takes the impact when the wheels meet the .