Plain English Meaning
The sky, treated as something with rules. It is split into zones, and each zone has its own requirements about who can fly there, what equipment they need, and how they must operate.
Full Definition
The volume of navigable atmosphere above a defined geographic area, divided by regulation into classes and special-use areas, each with specific entry requirements, equipment requirements, pilot qualifications, and operating rules.
Context Anchor
Pilots encounter airspace during flight planning, on aviation charts, near airports, during training flights, and whenever deciding where they may safely and legally fly.
Derivation
From 'air' plus 'space' — literally the space made up of air. The aviation use adds the idea that this space is mapped, classified, and regulated, not just open sky.