\Vac"u*um\, n.; pl. E. {Vacuums}, L. {Vacua}. [L., fr.
vacuus empty. See {Vacuous}.]
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 brake}, a kind of continuous brake operated by
   exhausting the air from some appliance under each car, and
   so causing the pressure of the atmosphere to apply the
   brakes.
{Vacuum pan} (Technol.), a kind of large closed metallic
   retort used in sugar making for boiling down sirup. It is
   so connected with an exhausting apparatus that a partial
   vacuum is formed within. This allows the evaporation and
   concentration to take place at a lower atmospheric
   pressure and hence also at a lower temperature, which
   largely obviates the danger of burning the sugar, and
   shortens the process.
{Vacuum pump}. Same as {Pulsometer}, 1.
{Vacuum tube} (Phys.), a glass tube provided with platinum
   electrodes and exhausted, for the passage of the
   electrical discharge; a Geissler tube.
{Vacuum valve}, a safety valve opening inward to admit air to
   a vessel in which the pressure is less than that of the
   atmosphere, in order to prevent collapse.
{Torricellian vacuum}. See under {Torricellian}.