Plain English Meaning
It is the way the sky is split into different zones, with each zone having its own rules about who can fly there, what equipment the aircraft needs, and how much help controllers give you.
Full Definition
The system the FAA uses to divide the airspace over the United States into categories — Class A, B, C, D, E, and G — each with its own rules for pilot certification, aircraft equipment, weather minimums, and the level of air traffic control service provided.
Context Anchor
Pilots meet airspace classification when reading charts, planning a flight, talking with air traffic control, and deciding what is required before entering a particular area of airspace.