Noun
a thermionic tube having five electrodes
Source: WordNetLater tetrodes had anodes treated to reduce secondary emission; earlier ones such as the type 77 sharp-cutoff pentode connected as a tetrode made better dynatrons. Source: Internet
To further reduce the cost and complexity of radio equipment, two separate structures (triode and pentode for instance) could be combined in the bulb of a single multisection tube. Source: Internet