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In view of the revival of interest in high-power magnetrons, an important magnetron parameter called the threshold voltage has been rederived for both relativistic and nonrelativistic cases using a novel and simplified approach. The main purpose of the paper is to investigate the physical significance of the threshold voltage and to show that, contrary to a long established opinion, no restriction on the amplitude of oscillations is required. In fact, the name itself is a misnomer, since in practice the threshold voltage is an approximate dc anode voltage at which the tube operates when delivering full power. It is this feature which, in our opinion, makes the threshold voltage so important and useful in practice. A set of numerical computations confirms our analysis and also reveals a form of Lorenz’s “butterfly effect” associated with the perceived start of oscillations.