In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional analog of "concentric".

Coaxial cable, for example, has a conducting wire in the center and a second conducting layer running all the way around the exterior circumference, under the insulation.

The term "coaxial" is also used by the military. In discussions of armored fighting vehicles, it denotes a machine gun or similar weapon mounted in a fixed orientation in the turret, immediately adjacent to and parallel with another weapon, typically the main gun of a tank, so that the machine gun is aimed by rotating the turret and elevating or depressing the main gun.