(originally) A person in training to be a (generally military) astronaut, especially in science fiction.

A toy consisting of a large inflated rubber ball upon which one bounces.

An unoccupied space where something may be stored / An unoccupied area or volume. / Vacuum; a space containing nothing at all

Heating of rooms within buildings.

(crystallography, mineralogy) The set of all symmetry operations that can be applied to a given crystal without changing it.

plural of space hopper

The period from the about the time of the launch of Sputnik in 1957 to the present day, during which people ventured into space. / (attributive) Redolent of the space age; appearing futuristic or modern.

The space in an electron tube in which there is no modulated field

Alternative spelling of airspace

(military) The areas between a weapon and its target in which the trajectory is not greater than the typical height of a human (about 1.8 meters)