Plain English Meaning
A chunk of sky with a defined top and bottom that a controller has cleared you to use. You can move up and down inside it freely, but you must stay within those limits.
Full Definition
A defined volume of airspace, typically described by a range of altitudes between two specified levels, within which an aircraft is cleared to operate. Air traffic control may issue a block altitude clearance authorizing flight anywhere between, for example, 10,000 feet and 12,000 feet, giving the pilot freedom to climb, descend, or maneuver within those limits without further clearance.
Context Anchor
Seen in instrument flying examples, ATC instructions, training , and descriptions where an aircraft must stay within a defined part of the sky.