Definition
A range of altitudes, between a stated lower and upper limit, within which an IFR aircraft is cleared by ATC to operate. The pilot may climb, descend, or maintain any altitude within that range without further clearance.
Plain English
Instead of being told to fly at one specific altitude, the controller gives you a band of altitudes — for example, between 8,000 and 10,000 feet — and you can fly anywhere inside that band.
Context Anchor
Seen in instrument clearances, procedure notes, and charted altitude instructions when a range is given instead of one specific altitude.
Derivation
Block here means a chunk or range of something set aside for a purpose, the same sense as in 'block of time' or 'block of seats.' The controller is reserving a chunk of the airspace vertically for that aircraft.
Why Pilots Care
It allows the pilot to adjust altitude for weather, turbulence, or traffic while preserving vertical separation from other aircraft.
Intuition Check
Block altitude does not mean an altitude that is blocked off or unavailable. It means an altitude range the aircraft is allowed to use.
Example Sentence 1
Unable to maintain smooth air, the crew requested a block altitude between 6,000 and 8,000 feet to ride above and below the chop.
Example Sentence 2
We used the block altitude to stay above the tops of the buildups without further coordination.