1.(Physics) A space entirely devoid of matter (called also, by way of distinction, absolute vacuum); hence, in a more general sense, a space, as the interior of a closed vessel, which has been exhausted to a high or the highest degree by an air pump or other artificial means; as, water boils at a reduced temperature in a vacuum.
2.The condition of rarefaction, or reduction of pressure below that of the atmosphere, in a vessel, as the condenser of a steam engine, which is nearly exhausted of air or steam, etc.; as, a vacuum of 26 inches of mercury, or 13 pounds per square inch.
Vacuum tube (Phys.), (a)a glass tube provided with platinum electrodes and exhausted, for the passage of the electrical discharge; a Geissler tube.
(a)any tube used in electronic devices, containing a vacuum and used to control the flow of electrons in a circuit, as a vacuum diode, triode, tetrode, or pentode.
