The term used to describe the period of time when neither the sun or moon are visible. This is most prevalent when there is an eclipse occurring which is not directly visible by you. Not to be confused with a new moon.
L - "God Richard, why can't I see the moon tonight?"
R - "Maybe It's foggy?"
L - "No I think it's a new moon"
R - "Just checked Google. There's an eclipse in Australia"