Lightspeed is defined as velocity divided by the speed of light, just as Mach number is velocity divided by the speed of sound. Light (in a vacuum) travels at lightspeed 1. Move at half the speed of light and you have lightspeed 0.5. The time dilation factor, the stretching that takes place when you compare time intervals in two reference frames, is called gamma (the Greek letter γ), and its formula is , where b is the lightspeed.
