Electricity—the size of the conductors used to carry electric current. In the United States, small wire gauges are referred to using the American wire gauge where 40-gauge wire is very small and 0000 ("Four-Oh") wire is the largest common size. Larger sizes of wires are referred to by their size in "Mean Circular Mils". In the rest of the world, wire gauges are referred to by their cross-section in square millimeters. Note that the smaller the gauge number, the larger the diameter.
A gauge in engineering can be any of a variety of measurement devices.
Jewelry - especially as applied to body piercing, gauge refers to the thickness of the metal that penetrates the body tissue. As a point of reference, the holes in pierced ears normally exhibit diameters of 20 or 18 gauge, using a scale similar to that used for electric wire.
Railway - track or rail gauge means the distance between the inside edges of the two rails forming the track (for example, standard gauge and narrow gauge) while loading gauge refers to the set of height and width profiles governing the maximum dimensions of railway vehicles.
Shotgun - the diameter (caliber) of the barrel. The gauge is determined by the number of solid spheres of a diameter equal to the inside diameter of the barrel that could be made from a pound of lead. The term bore is also used for this, especially in British English. It is more rarely used to refer to the internal dimensions of a cannon.
