A region of space devoid of any fluid molecules resulting in a zero pressure in ideal cases. In practice, even space has a small but finite number of molecules. In QFT, it signifies the ground state devoid of excitations/particles.
Strictly speaking, vacuum is a region of space without any particles. In this sense it is used in the Quantum Field Theory where it means the ground state of the quantum field.
This rather theoretical requirement is never achieved in practice, because even if space does not contain any atoms / electrons / nucleons, it does contain a lot of photons and neutrinos. But we still call it a vacuum, as an approximation of the theoretical vacuum. Interplanetary or interstellar space is a relatively good example of vacuum, even though there is still some small, but non-zero amount of hydrogen and other elements / particles.
