sidelight

noun

1. A light found at the side of something; especially of a vehicle.

   “If it starts getting misty, put your sidelights on.”

   “After dark the train is a lighted snake, as, even when the passengers' lights are out, each carriage has a side-light in the middle just under the eaves.”

2. (broadly)A light found at the side of something; especially of a vehicle.
3. A window found at one or both sides of a door.
4. A piece of incidental information that helps one understand a subject.

   “A book of this type is necessarily mainly factual, but it is diversified by many incidents and sidelights which help to give life to a story now accorded its due place in railway history.”

   “He is well up to form when dealing with passenger stock, notably Victorian and Edwardian and on this and on East Coast motive power he throws many sidelights.”