Explanation: Plate saturation describes the condition where, for a given filament or cathode temperature (and thus a fixed rate of electron emission), increasing the plate voltage no longer increases the plate current. At this point, the plate current has reached its maximum because all electrons emitted by the cathode are being attracted to and collected by the plate. The plate current precisely equals the total electron emission. "Electron saturation" is not a standard term in this context. "Filament saturation" (or cathode saturation) refers to the maximum number of electrons the filament *can* emit, which is determined by its temperature. Plate saturation, however, describes the point where the *plate current* has drawn *all* of the already emitted electrons.