Gas-discharge tube and Thermionic tube

Gas-discharge tube noun - A tube in which an electric discharge takes place through a gas.

Thermionic tube and gas-discharge tube are semantically related. In some cases you can use "Thermionic tube" instead a noun phrase "Gas-discharge tube".

Thermionic tube noun - Electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope.

Gas-discharge tube and thermionic tube are semantically related. Sometimes you can replace phrase "Gas-discharge tube" with "Thermionic tube", this phrases are similar.