Airspace is the space of air above the ground or water on earth. Duh.
Of course, when you are talking about an air space you are generally talking about the space of air above a particular chunk of land or water. For example, countries often dictate which other country’s planes can fly over their airspace and whether the planes flying through their airspace have to pay a fee for that honor. Because that’s the way countries are.
Owners of land might sell airspace rights, also called air rights, above their properties. In that case, the landowners keep title to their land, but they sell the right to build over it to someone else.
Generally, the people who buy air rights expect to have the right to put some supports for their building on the land they didn’t buy, Either that or they expect to be able to rest their building on the landowner’s building that is already sitting there. If the people who buy the air rights don’t also get that right then, in legal terms, they are referred to as morons.