mid 16th century: modern Latin, neuter of Latin vacuus ‘empty’.

1. a space entirely devoid of matter.

2. an enclosed space from which matter, especially air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere (opposed to plenum).

3. the state of a gas at which its pressure in a vessel and therefore its particle density is lower than that of the ambient surrounding atmosphere or in which the pressure of the gas is lower than 300 mbar.

4. any volume of space where pressure is less than standard sea-level pressure—that is, atmospheric pressure of 29.92 in [760 millimeters]) of mercury (or, one atmosphere [1 atm]).

5. a state of isolation from outside influences.

6. (physics) a spacetime having tensors of zero magnitude.