Plain English Meaning
VNAV is the part of the navigation system that handles climbing and descending. Just as the lateral side of the system tells the aircraft where to go left and right, VNAV tells it how to climb or descend along a planned vertical path between two points.
Full Definition
A function of area navigation (RNAV) equipment that calculates, displays, and may guide the aircraft along a vertical flight profile, typically a defined descent path between two altitudes at specified points along a route or instrument approach.
Context Anchor
Seen in aircraft navigation systems, flight management systems, instrument procedures, descent planning, and some operations.