Plain English Meaning
A flying machine that stays up because air flows over its wings. If the air stops moving across the wings, the lift goes away and it can no longer fly.
Full Definition
An aircraft that depends on the movement of air over its lifting surfaces to generate the lift required to support its weight in flight. An airplane is an aerodynamic vehicle because it must maintain forward airspeed for its wings to produce lift; it cannot remain airborne without that airflow.
Context Anchor
Seen in basic flight discussions when describing how an airplane is supported and controlled in the air.
Derivation
From Greek 'aer' (air) and 'dynamis' (power or ). An aerodynamic vehicle is one that gets its supporting force from the action of air -- specifically, air moving across its surfaces.