1. In general, the maximum current which can be obtained under certain conditions.

2. In a vacuum tube, the space-charge-limited current, such that further increase in filament temperature produces no specific increase in anode current.

3. In a vacuum tube, the temperature-limited current, such that a further increase in anode-cathode potential difference produces only a relatively small increase in current.

4. In a gaseous-discharge device, the maximum current which can be obtained for a given mode of discharge.

5. In a semiconductor, the maximum current which just precedes a change in conduction mode.

The ionization current in a gas tube when the applied potential is large enough to collect all ions produced by ionizing radiation.