suspended in mid-concept

“Planes collide in midair” — raising the question: what are the boundaries of midair anyway? Is it midair if they’re one foot off the ground? One hundred? If there are mountains nearby, do they make a difference? When does a descending spaceship reach midair? In short, does midair tell us anything that air would not? Midocean can be defined as out of sight of land, or beyond the continental shelf, or in international waters; but none of these has an obvious analogy for air.