Light

From Memory Alpha, the free Star Trek reference

Light is electromagnetic radiation that is detectable to the eye. Occasionally, the term light was used to refer to electromagnetic radiation in general. Light could be understood quantitatively by a unit called a 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.