A variable speed of light (VSL) is a feature of a family of hypotheses stating that the speed of light in vacuum, usually denoted by "c", may in some way not be constant, e.g. varying in space or time, or depending on frequency. Accepted classical theories of physics and in particular general relativity predict a constant speed of light in a local frame of reference and in some situations these predict apparent variations of the speed of light depending on frame of reference, but this article does not refer to this as a variable speed of light. In various alternative theories of gravitation and cosmology, many of them non-mainstream, incorporate variations in the local speed of light.