Light is electromagnetic radiation that is detectable to the eye. Occasionally, the term "light" was used to refer to electromagnetic radiation in general. A quantitative unit of light is the photon.
The distance that light travels in one year was a unit of measurement for starship navigation and the speed that light travels was a unit of measurement for starship velocity.
Some cloaking and holographic technologies operated by manipulating light in some fashion. (ENT: "In a Mirror, Darkly"; VOY: "Phage", "Repentance")
Light could be created from a variety of different sources including light bulbs, fires, candles, and stars.