Real Dictionary

light brightness or illumination
Light is the brightness that lets you see things. Light comes from sources such as the sun, moon, lamps, and fire.
countable noun: A light is something such as an electric lamp which produces light.
plural noun: You can use lights to refer to a set of traffic lights.

light in American English 1
the form of electromagnetic radiation that acts upon the retina of the eye, optic nerve, etc., making sight possible: this energy is transmitted in a vacuum at a velocity of 299,792,458 meters per second (c. 186,000 miles per second)
noun: something that makes things visible or affords illumination

light in American English 2
having little weight; not heavy
adjective: of little weight; not heavy

light in American English 3
intransitive verb: to get down or descend, as from a horse or a vehicle

light in Automotive Engineering
noun: A light is a window in a passenger vehicle.

Light in British English
noun: God regarded as a source of illuminating grace and strength

light in British English 1
noun: the medium of illumination that makes sight possible

light in British English 2
adjective: not heavy; weighing relatively little

light in Chemical Engineering
noun: A light is a component which has a lower boiling temperature in a distillation process.

light not great in weight, amount, or intensity
adjective: Something that is light does not weigh very much, or weighs less than you would expect it to.

light unimportant or not serious
If you describe things such as books, music, and films as light, you mean that they entertain you without making you think very deeply.
If you say something in a light way, you sound as if you think that something is not important or serious.