A small, dim light or lamp left on overnight(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "nightlight" in a sentence
"Because, really, picking a nightlight is a struggle between it being bright enough to provide reassurance and visibility, and dim enough to allow sleep ...."
"But as "nightlight" stations, they'll now be broadcasting a digital TV message on a loop for the next few weeks, explaining to people why they're no longer receiving their regular TV."
"a boy tries to fall asleep. his nightlight is a myth that burns out once a day"