**Gauge invariance / Gauge symmetry / Gauge theory** Gauge invariance, also called gauge symmetry, is the property of a field theory in which different configurations of the underlying fields — which are not themselves directly observable — result in identical observable quantities.

A theory with such a property is called a gauge theory.

A transformation from one such field configuration to another is called a gauge transformation.

__Source:__ http://en.wikipedia.org/wiki/Introduction_to_gauge_theory