Coaxial

Coaxial means sharing a common axis, and in defense the word attaches to three unrelated pieces of hardware. A coaxial machine gun on a tank. A coaxial rotor system on a helicopter. Coaxial cable in electronics. They have nothing in common but the geometry the word describes, two things aligned on the same axis, so which one is meant depends entirely on whether you are talking about tanks, helicopters, or wiring.

The tank's coax

The most common defense usage is the coaxial machine gun, universally shortened to coax. It is a machine gun mounted alongside a tank's or fighting vehicle's main gun, aligned on the same axis and slaved to the same aiming system, so it points wherever the big gun points. The purpose is economy of effect: a tank does not want to spend a main-gun round, expensive, limited in number, on infantry in the open or a soft target, so the gunner uses the coax for those and saves the main gun for armor and hard points. It also serves as a ranging and targeting aid and handles the close-in threats a slow-traversing main gun cannot. Nearly every tank in service, the Abrams, Leopard, Challenger, and their peers, carries a coax, typically a 7.62mm machine gun, and it is one of the most-used weapons on the vehicle.

The helicopter's coax

A coaxial rotor system uses two rotors stacked on the same mast, turning in opposite directions. This cancels the torque that would otherwise spin the fuselage, which is what a conventional helicopter's tail rotor exists to counter, so a coaxial design needs no tail rotor and gains a more compact airframe and, often, higher speed. The Russian Kamov helicopters, the Ka-50 and Ka-52 attack aircraft, are the signature examples, and the configuration reappeared in high-speed Western concepts like Sikorsky's designs. It buys performance at the cost of mechanical complexity in the rotor head.

The cable

Coaxial cable, the electronics sense, is a wire surrounded by a tubular shield on the same axis, which contains high-frequency signals and blocks interference. It is everywhere in defense electronics, carrying radar and radio signals between antennas and equipment, and while unglamorous, the milspec versions rated for the frequencies and environments defense systems demand are their own small specialty market.

So how do you know which coaxial is meant?

Context, always. Tanks and fighting vehicles: the machine gun. Helicopters and rotorcraft: the stacked-rotor system. Radar, radios, and electronics: the cable. The word is precise about geometry and silent about everything else, so the surrounding subject carries the meaning.