When the Moon's illumination is decreasing with each passing day, it is said to be waning.

Here's why:

* Waxing: The Moon's illumination increases daily, moving from a thin crescent to a full disk.

* Waning: The Moon's illumination decreases daily, moving from a full disk back to a thin crescent.

Think of it like a "waxing" candle getting bigger and brighter as the wax melts, and a "waning" candle getting smaller and dimmer as the wax burns away.