Plain English Meaning
The pushing-back force the air creates against the airplane as it moves forward. Anything sticking out into the airflow, like the landing gear, adds to it.
Full Definition
The rearward force exerted on an airplane by the air it is moving through, opposing the airplane's forward motion. In the context of landing gear, extending the gear into the airstream increases this force and helps slow the airplane.
Context Anchor
In the Airplane Flying Handbook landing gear section, you will see this idea when fixed or extended landing gear creates extra air resistance and affects airplane performance.
Derivation
From Greek 'aero' (air) and Latin 'dynamicus' (relating to force or motion). 'Drag' is plain English for something that pulls or holds back. Together: a force from the air that holds the airplane back.