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.
Plain English
The leg between the wheel and the airplane that holds the wheel in place and cushions the bumps when you land or taxi.
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 weight off the ground and takes the impact when the wheels meet the runway.
Why Pilots Care
A functioning strut prevents damage to the airframe by absorbing impact forces and keeps the aircraft stable on the ground.
Analogy
It works somewhat like a shock absorber on a car: it supports the vehicle and helps keep bumps or hard contact from being passed directly into the frame.
Intuition Check
Do not read “strut” here as a way of walking or showing off. In this aviation use, a strut is a support part that helps hold weight and absorb landing or taxi loads.
Example Sentence 1
During preflight, the pilot checked each landing gear strut for proper extension and looked for any sign of hydraulic fluid leaking from the seals.
Example Sentence 2
After a firm touchdown the strut compressed to absorb the impact and keep the fuselage level.