sidelight

side +‎ light

sidelight (plural sidelights)

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

   If it starts getting misty, put your sidelights on.

   • 1950 April, Timothy H. Cobb, “The Kenya-Uganda Railway”, in Railway Magazine, page 263:

     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. A window found at one or both sides of a door.
3. A piece of incidental information that helps one understand a subject.
   • 1962 October, “New Reading on Railways: The Flying Scotsman 1862-1962. By C. Hamilton Ellis. Allen & Unwin. 6s.”, in Modern Railways, unnumbered page:

     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.

• “sidelight”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.