sidelight

noun4

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

   • If it starts getting misty, put your sidelights on.
2. •
   (nautical) Red and green navigation lights indicating each side of a ship for collision avoidance; red on the port (left) and green on the starboard.
3. •
   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. — 1957
4. •
   A window found at one or both sides of a door.