The number of light flashes per second at which the light is percieved to be continuous. At some point (for people usually greater than 10 Hz), the photoreceptors in our eyes do not have time to recover from a previous flash to sense a new one. Hence the photoreceptor is continously depolarized and the light is perceived as continuous. This is why lights powered by household alternating current (60 Hz) do not appear to flash and also why the television and movies appear continuous.