Definition
A specific altitude that ATC requires an aircraft to be at when it passes over a designated point, fix, or navaid along its route. The clearance specifies both the location and the altitude that must be met at that location.
Plain English
The height ATC tells you to be at when you fly over a particular spot.
Context Anchor
You will see this in clearances for flying by instruments, especially during arrivals, departures, and route changes.
Derivation
Crossing comes from the idea of passing across or through a point. Altitude comes from the Latin altus, meaning high. Together, the phrase points to the required height as you pass a specific place.
Why Pilots Care
Maintains required vertical separation from terrain, obstacles, and other aircraft at critical route points.
Intuition Check
Do not read crossing altitude as just the altitude you happen to have while passing somewhere. In this context, it is a required altitude tied to a specific point in the clearance.
Example Sentence 1
ATC issued the clearance 'cross LENDY at 11,000,' so the crew began their descent early enough to meet the crossing altitude.
Example Sentence 2
Cross the VOR at or above 5000 feet as specified in the departure clearance.