Plain English Meaning
The middle part of a flight — when you've finished climbing away from where you took off and haven't yet started coming down to land. You're on your way.
Full Definition
The phase of flight between departure and arrival, when the aircraft is established along its planned route, typically at cruise altitude, after the climb-out from the departure airport and before beginning the approach to the destination.
Context Anchor
Seen in instrument flying when discussing en route charts, navigation, weather, and air traffic services during the middle portion of a flight.