Did a quick web search on it, and came up with this definition of Lorentz factor. The Mount Allison University Special and General Relativity Electronic Text looks pretty interesting, so thanks for that.

The speed of light depends upon the curvature of space, in general relativity. In flat space, where special relativty is valid, it is limited--otherwise, energy from one event could reach a second event in the first one's future, and energy from the second event could reach the first one in the second one's future!