earthshine, sunlight reflected from the Earth, especially that reflected to the Moon and back again. For a few days before and after New Moon, this doubly reflected earthshine is powerful enough to make the whole Moon visible.
At this time an observer on the Moon would see the Earth as a bright body, four times the diameter of the Moon as seen from Earth, almost completely illuminated by the Sun. The phases of the Earth and Moon are complementary, so that the Earth is near full when the Moon is near new, and the earthshine then is strongest.