noun
1. : an enclosed space for or containing air: as
a. : space (as in a room) available for air for respiration
b. : a space between walls or in a wall to protect against dampness or to provide thermal insulation
c. : air sac 2a
d. : the space between the powder charge and projectile in a firearm
e. : a cavity containing air in the cellular tissue of a plant
2. usually airspace : the space lying above the earth or above a certain area of land or water ; especially : the space lying above a nation and considered as under its jurisdiction