The moon is waning when it is getting smaller
Waning.
Then the Moon is "waning".
The correct term is a 'WANING Moon. From Full Moon to the next New Moon, the moon is said to be WANING. From Full Moon to Last Quarter is it a WANING GIBBOUS Moon. From Last Quarter to the next New Moon it is a WANING CRESCENT Moon. In very clear weather you will see the waning crescent Moon, with the 'ghost' of the Moon's full circle. In old lore it is said to be 'The old moon, with the new moon in her arms'.
waning
waning
Waning.
Waning.
It is said to be waning, and waxing when the moon's light is increasing.
Waxing
Waning gibbous, Third Quarter and Waning Crescent are the 'waning' moon phases.
Waning, or waning gibbous.
When the moon appears to be decreasing in size, it is called waning. When the moon appears to be increasing in size, it is called waxing.