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New for August 2023. As a microwave transmission line, triax cables are sometimes used to suppress electro-magnetic interference (EMI). We'll leave that topic for another day.
If you work in a semiconductor lab, you might encounter triax cables and connectors. Triax is a portmanteau of triple and coax. Triax is used for the most sensitive measurements in place of coax. Coax typically has Teflon dielectric, which is close to a perfect insulator. However, it does leak some current from the center conductor to the outer conductor. In the microwave circuit model, the is the G' term we ignore 99.9% of time, the conductivity across the transmission line per unit length.
In a common laboratory application of triax, it is used to bring DC current/voltage from specialized test equipment (such as semi-conductor analyzers), to sensitive devices. Keysight, Keithley and other suppliers make equipment that uses triax interfaces.
Laboratory triax connectors are similar to dime-a-dozen BNC connectors. Regarding the lowly lab-standard BNC coax connector, its center-conductor is referred to as the "force" and its outer conductor as the "shield". Triax has a special nomenclature you should know. The center-conductor is still called the force, the outer conductor is still called the shield, but the in-between jacket is referred to as the "guard".
In the video below, Keysight engineer Grant Suter explains how triax is used to measure atto-amperes (!) of current without leakage errors, and follows up with some advice, including safety precautions, for mating BNC connectors to triax connectors using specialized adapters.
Keysight's Grant Suter explains triax