Another name for the new moon, when the moon is completely in the Earth's shadow. Astrology interprets the new moon as a time of reflection and new beginnings.

Dark Moon and Grief

from Astrology.com

Whenever we experience a deep loss, we are changed profoundly, which is a kind of death. This is considered a dark Moon phase, and lasts as long as it takes to fully integrate the experience. Sometimes others are made uneasy by our personal confusion, melancholy, soul angst, etc, and try to prevent us from fully dwelling in the dark. But taking a cue from nature, we can see that everything dies for a time, before coming alive again in a new form. Just like that, there are times when we die to our old self and are reborn to a new life.