Plain English Meaning
A support rod that keeps a landing gear leg from being pushed backward when the wheels hit the ground or when the brakes are applied. It ties the gear leg to the airframe so the gear stays where it should.
Full Definition
A structural member in a landing gear assembly that resists fore-and-aft (drag) loads imposed on the gear during landing rollout, taxi, and braking. The drag strut connects the main gear leg to the airframe at an angle, transferring rearward forces into the aircraft structure and helping hold the gear in its extended or retracted position.
Context Anchor
Seen in landing gear system descriptions, discussions, and of exposed landing gear components.