IN THEORY: Light speed is the rate of travel of light in an un-interupted vacuum. Exactly 299,792,458 metres per second (m/s), where metre is difined by 1983 Systeme International (SI) as "The length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.". The theory of realitivity describes light speed as impassible in rate of travel, but this is based on the fact it hasn't been done yet *rolls eyes*.
However, Light Speed is acctually better told as such: The speed of light is the commonly used interval of time it takes for the green light to turn amber on a traffic light in South Essex, UK, or the time it takes for a traffic warden to travel from the end of the street to the parking spaces to give you a ticket as soon as you pull in.
