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]

WordNet (r) 3.0 (2006):

vacuum tube
    n 1: electronic device consisting of a system of electrodes
         arranged in an evacuated glass or metal envelope [syn:
         tube, vacuum tube, thermionic vacuum tube,
         thermionic tube, electron tube, thermionic valve]

The Free On-line Dictionary of Computing (30 December 2018):

electron tube
firebottle
vacuum tube

    (Or tube, vacuum tube, UK: valve, electron
   valve, thermionic valve, firebottle, glassfet) An electronic
   component consisting of a space exhausted of gas to such an
   extent that electrons may move about freely, and two or more
   electrodes with external connections.  Nearly all tubes are of
   the thermionic type where one electrode, called the cathode,
   is heated, and electrons are emitted from its surface with a
   small energy (typically a Volt or less).  A second electrode,
   called the anode (plate) will attract the electrons when it is
   positive with respect to the cathode, allowing current in one
   direction but not the other.

   In types which are used for amplification of signals,
   additional electrodes, called grids, beam-forming electrodes,
   focussing electrodes and so on according to their purpose, are
   introduced between cathode and plate and modify the flow of
   electrons by electrostatic attraction or (usually) repulsion.
   A voltage change on a grid can control a substantially greater
   change in that between cathode and anode.

   Unlike semiconductors, except perhaps for FETs, the
   movement of electrons is simply a function of electrostatic
   field within the active region of the tube, and as a
   consequence of the very low mass of the electron, the currents
   can be changed quickly.  Moreover, there is no limit to the
   current density in the space, and the electrodes which do
   dissapate power are usually metal and can be cooled with
   forced air, water, or other refrigerants.  Today these
   features cause tubes to be the active device of choice when
   the signals to be amplified are a power levels of more than
   about 500 watts.

   The first electronic digital computers used hundreds of vacuum
   tubes as their active components which, given the reliability
   of these devices, meant the computers needed frequent repairs
   to keep them operating.  The chief causes of unreliability are
   the heater used to heat the cathode and the connector into
   which the tube was plugged.

   Vacuum tube manufacturers in the US are nearly a thing of the
   past, with the exception of the special purpose types used in
   broadcast and image sensing and displays.  Eimac, GE, RCA, and
   the like would probably refer to specific types such as "Beam
   Power Tetrode" and the like, and rarely use the generic terms.

   The cathode ray tube is a special purpose type based on
   these principles which is used for the visual display in
   television and computers.  X-ray tubes are diodes (two element
   tubes) used at high voltage; a tungsten anode emits the
   energetic photons when the energetic electrons hit it.
   Magnetrons use magnetic fields to constrain the electrons;
   they provide very simple, high power, ultra-high frequency
   signals for radar, microwave ovens, and the like.  Klystrons
   amplify signals at high power and microwave frequencies.

   (1996-02-05)