the.com/night light

means A small, dim lamp left on overnight so a room is never fully dark.

from A plain compound of 'night' and 'light,' English's favorite trick for making new words — just push two old ones together. 'Night' goes back through Old English 'niht' to a very old Germanic and Indo-European root for darkness, and 'light' has its own ancient pedigree meaning brightness or illumination. The phrase itself seems to have taken on its specific domestic sense — the small bedside or hallway lamp kept burning for comfort or safety — as electric lighting became common in homes, giving the dark a gentle, affordable opponent.