Plain English Meaning
The speed the aircraft is flown at once it has reached its planned altitude and is travelling level toward its destination, before it begins to descend.
Full Definition
The airspeed at which an aircraft is flown during the en route, level portion of a flight, selected by the pilot based on the aircraft's performance, fuel efficiency, and operational requirements. It is the steady airspeed maintained between the climb and descent phases.
Context Anchor
In this section, you see it when the handbook explains how to enter a straight climb or descent from normal level flight.