Term: "Unoccupied Space"

A space, often a simple normal 5 ft x 5 ft square, is unoccupied if no creatures are in it and it isn’t filled with other stuff. Of course, this can get heady, but usually, most things follow a simple rule.
A 5 by 5 space is usually enough to put a modern, sensibly sized car in. If something is in it, and you can't also put a car in it, then that space is not Unoccupied.
Examples include: A table, another creature, a baby, a ton of bricks, a ton of feathers, etc