Meaning : A gradual decrease in magnitude or extent.
Example :
The waning of his enthusiasm was obvious.
The waxing and waning of the moon.
Meaning : (of the Moon) pertaining to the period during which the visible surface of the moon decreases.
Example :
After full moon comes the waning moon.
(of the moon) pertaining to the period during which the visible surface of the moon increases.
The waxing moon passes from new to full.