A gauge for measuring the diameter of wire, usually consisting of a disk having variously sized slots in its periphery or a long graduated plate with similar slots along its edge.
A standardized system of wire sizes.
An instrument for measuring the diameter of wire, thickness of sheet metal, etc.: it usually consists of a disk with notches of graduated sizes around its edge.
A measurement of the thickness of a cable, which is the inverse of its size. For example, the diameter of an 18 gauge wire is 1.02mm, while the smaller number 12 gauge is 2.05mm, more than twice the diameter. This relationship makes more sense if you think of how many wires fit into a given area. For example, if the thickness of 18 wires fit into the same space as 12 wires, the thickness of the 18 wires has to be smaller.