Decreasing, declining. The period of reduction from the Full Moon to the New moon.
Referring to the time when the Moon, moving from full Moon to new Moon, is closer and closer to the Sun each day, and lit up less and less.
a gradual decrease in magnitude or extent; "the waning of his enthusiasm was obvious"; "the waxing and waning of the moon"
(of the moon) pertaining to the period during which the visible surface of the moon decreases; "after full moon comes the waning moon"
when the moon goes from a full moon to a new moon (the opposite of waxing).
the decreasing of the moon's visible illuminated surface, from full moon to new moon.
the period between full moon and new moon