(physiology) The portion of the respiratory tract that is ventilated but not perfused by pulmonary circulation.

Ordinary two- or three-dimensional space, characterised by an infinite extent along each dimension and a constant distance between any pair of parallel lines.

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

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

(physics) vacuum; a space free of matter. / Used other than figuratively or idiomatically: see free, space.

the totality of information, especially that which is accessible online

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

(mathematics) Euclidean space described by Cartesian coordinates.

(chemistry) the set of all possible stable molecules

(mathematics) The vector space generated by all the column vectors of a matrix