In engineering, gauge refers to a measure of width or thickness, or to devices used to make measurements (originally of width or thickness). It can also be used as a verb to describe the act of taking a measurement, typically an estimate.
Gauge is most commonly used when referring to railways, where gauge (commonly misspelt as guage) referrs to the distance between the two rails of the roadbed (eg standard gauge, narrow gauge).
The term is also used in the measurement of metal sheeting, where it refers to the thickness of the sheet.
It is more rarely used to refer to the internal dimensions of a cannon.
In mathematics and physics, a gauge transformation is a member of a group of mappings to a space or a spacetime, where this group of mappings satisfies certain properties. The Lagrangians of bosons, which mediate interactions between fermions, in the theories of the electroweak interaction and quantum chromodynamics of the Standard Model of particle physics, are invariant under gauge transformations, so these bosons are called gauge bosons.
See also rail gauge.