What is define waning?

Waning refers to the phase of the moon in which the illuminated portion appears to decrease gradually from a full moon to a new moon. It is the opposite of waxing when the illuminated portion appears to increase. The waning phase of the moon is caused by the relative positioning of the moon, Earth, and the sun, as the moon rotates around the Earth. Waning can also be used more generally to describe a decline or decrease in quantity, intensity, or quality over time.