Definition
Flight levels ending in 000 -- specifically 5,000; 6,000; 7,000 feet, and so on -- as opposed to intermediate levels such as 4,500 or 6,500 feet. The term is used when ATC or procedures require an aircraft to fly at one of these even-thousand reference altitudes rather than at an odd-numbered or half-thousand altitude.
Plain English
Round-number altitudes that end in three zeros, like 5,000 or 8,000 feet, instead of altitudes like 5,500 or 7,500.
Context Anchor
You may see this term in ATC instructions, altitude assignment discussions, or FAA guidance about using standard flight levels.
Derivation
Cardinal comes from the Latin cardinalis, meaning 'principal' or 'main' -- the same root behind cardinal numbers (one, two, three) and cardinal directions (N, S, E, W). In aviation, cardinal flight levels are the 'principal' altitudes on the thousand-foot grid, with non-cardinal levels sitting between them.
Why Pilots Care
These levels are assigned by ATC to maintain orderly vertical separation between opposing traffic flows.
Intuition Check
Cardinal does not mean north, south, east, or west here. It means the main whole-thousand-foot flight levels, such as FL 250 or FL 260.
Example Sentence 1
ATC cleared the flight to climb and maintain 9,000, a cardinal flight level.
Example Sentence 2
Westbound traffic is usually stepped up through cardinal flight levels ending in zero.