The side of a planet that faces away from the sun around which it orbits(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "nightside" in a sentence
"Though the somewhat misnamed 'nightside' of the satellite was still permanently frozen, an area as large as Africa now had a temperate climate, liquid water, and a few scattered islands."
"A man like Titus Oates occurs like a slip of the tongue, disclosing the unconscious forces, the nightside of an age ..."
"And my gift to him is that part of him lives always here, the nightside of the city that shaped him - and that he is always, always loved."