A glass or metal envelope, evacuated but with a gas at low pressure introduced for special purposes, containing a heated cathode from which electrons are emitted and with an anode for collecting some or all of these electrons. Additional electrodes may be present to control the flow to the anode.
One such is the 572B triode (or T160L) shown here, manufactured by the Cetron Electronic Corporation of Geneva, Illinois, USA and in their terms called an "electronic tube".

Another is the 6146B (or 8298A) made by RCA of Lancaster, Pennsylvania, USA. and shown below.