The OED defines “mid-air” (it uses the hyphen) as “a part or region of the air not close to the ground (or some equivalent surface).”

How do you write midair?

So as a conclusion I would say midair and mid-air are both correct, but mid air is not. a point in the air, not on the ground: She caught the ball in midair. She caught the ball in mid-air.

How do you use midair in a sentence?

Midair sentence example

They stood in front of an airy, light tent resembling a silk sheet suspended in midair over a table. Toby laughed and tossed Rhyn another pillow, delighted when he snatched it from midair and shredded it in an explosion of white feathers.