Plain English Meaning
How long the flight will take from takeoff to arrival over the destination, based on how fast the airplane is actually moving across the ground.
Full Definition
The total time required to fly from the departure point to the destination, calculated by dividing the distance to be flown by the groundspeed.
Context Anchor
Used in flight planning and wind-triangle calculations after the pilot knows the distance to be flown and the speed the aircraft will make over the ground.
Derivation
From the French 'en route', meaning 'on the way'. Time enroute is literally the time spent on the way between two points.