A cathode is an electrode whose role depends on the device. In a vacuum display such as a Nixie tube or vacuum fluorescent display, it can emit electrons; in other devices, it may collect them. In a Nixie tube, shaped cathodes glow to form digits when the tube is energized.
Cathodes are not cybersecurity controls, but understanding them helps when inspecting, restoring, or maintaining hardware that uses older display technologies. Their condition and electrical connections affect whether the display works as intended. In security-sensitive equipment, accurate component identification can support hardware audits and troubleshooting, while unusual changes to physical components may warrant investigation.