Plain English Meaning
The highest altitude you are allowed to fly on a particular route segment. Going above it is not permitted on that segment.
Full Definition
A published altitude representing the highest usable altitude for an airspace structure or route segment. It is the maximum altitude at which an aircraft may be operated on that segment, established because of design limitations, frequency protection, airspace restrictions, or other operational factors.
Context Anchor
Seen on IFR charts and route information when planning or flying along published airways or route segments.
Derivation
Maximum from Latin maximus meaning greatest; altitude from Latin altus meaning ; indicating official permission. Together they specify the greatest height controllers are allowed to assign on that route.