Earthlight is the partial illumination of the dark portion of the moon's surface by light reflected from the earth. It is also known as earthshine, the Moon's ashen glow or the old Moon in the new Moon's arms.
This phenomenon is best observed just after new moon, when the moon is a thin crescent. On these nights it is easy to see the entire disk of the moon. The word is rare but is recognised by the Oxford English Dictionary.
It would also be a suitable term for an observer on the moon seeing the Earth during the lunar night. Arthur C. Clarke uses it in this sense in his novel Earthlight.