Space refers to the space between celestial bodies. The atmospheres of solid and gaseous celestial bodies (such as stars and planets) do not have a fixed boundary upward, but gradually become thinner as the distance to the celestial body increases. Above a certain height, one speaks of the beginning of outer space.

In space there is a high vacuum with low particle density. However, it is not an empty space, but contains gases, cosmic dust and elementary particles (neutrinos, cosmic rays, particles), furthermore electric and magnetic fields, gravitational fields and electromagnetic waves (photons). The almost complete vacuum in space makes it extraordinarily transparent and allows observation of extremely distant objects, such as other galaxies. However, nebulae of interstellar matter can also greatly obstruct the view of objects behind them.

Exploration of outer space is called space exploration. Travel or transportation into or through space is called space travel.