Definition
An ATC altitude restriction requiring an aircraft to cross a specified fix at or below a stated altitude. The pilot may cross the fix at any altitude from the maximum stated altitude down to the minimum IFR altitude for that segment of flight.
Plain English
When ATC tells you to cross a point 'at or below' a certain altitude, you must be at that altitude or lower as you pass over it — but never lower than the minimum safe altitude allowed for that route.
Context Anchor
Used in ATC clearances and on arrival, departure, or approach procedures when a pilot must meet an altitude limit at a named point.
Derivation
Altitude comes from the Latin word altus, meaning high. Cross, in this use, means to pass a point along your route. Fix means a fixed position used for navigation, so the phrase points to your height limit when you pass that specific place.
Why Pilots Care
Maintains required vertical separation from other traffic and ensures terrain clearance along the route.
Grounding Statement
At the named point, your altitude must be no higher than the assigned limit.
Intuition Check
Do not read “at or below” as “try to be below.” It means the assigned altitude is the highest altitude allowed at that fix; lower is acceptable only when the rest of the clearance and safe terrain clearance allow it.
Example Sentence 1
Cleared to cross WEMER at or below 8,000 — the pilot planned a descent that arrived at the fix at 7,500 feet, well above the minimum IFR altitude.
Example Sentence 2
We planned our descent to meet the 'cross JAX at or below FL180' restriction.