A gas discharge tube (GDT) is a type of circuit protection device that is used to protect electronic circuits from overvoltage transients, such as lightning strikes and electrical surges. A GDT consists of a sealed glass or ceramic tube filled with a gas mixture, such as argon and nitrogen. When an overvoltage transient occurs, the voltage across the GDT exceeds the breakdown voltage of the gas, causing an electric arc to form within the tube. The arc acts as a short circuit, rapidly dissipating the excess energy and protecting the circuit from damage. GDTs are often used in conjunction with other types of circuit protection devices, such as metal oxide varistors (MOVs) and surge suppressors, to provide a robust and effective protection against overvoltage transients.