speed of light
- noun the constant speed at which light and all electromagnetic radiation travels through a vacuum, equal to 2.998 × 108 metres per second
- The speed at which light propagates through a vacuum. This speed is a physical constant currently defined as 2.99792458 × 108 meters per second. electromagnetic waves of all frequencies travel at the same speed in a vacuum. Its symbol is c.
- The speed at which light propagates through a given medium. It is generally less than its speed in a vacuum.