side·light / ˈsīdˌlīt/ • n. 1. a light placed at the side of something. ∎ (sidelights) a ship's port (red) and starboard (green) navigation lights. ∎ fig. a piece of incidental information that helps to clarify or enliven a subject. ∎ natural light coming from the side.2. a narrow window or pane of glass set alongside a door or larger window.