The Collaborative International Dictionary of English v.0.48:

Vacuum \Vac"u*um\ (v[a^]k"[-u]*[u^]m), n.; pl. E. Vacuums
   (v[a^]k"[-u]*[u^]mz), L. Vacua (v[a^]k"[-u]*[.a]). [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.
      [1913 Webster]
      [1913 Webster]

   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.
      [1913 Webster]

   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) 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.

   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.
      [1913 Webster]