What is the word used to describe the fraction of light that is reflected off the moon?

The answer is Moon Phase.

Explanation:

The moon phases are the changes in the portion of light seen on the satellite due to the position with respect to the Earth and the sun.

Its lighted portion decreases for 2 weeks and then increases again for 2 weeks.

Each portion of light illuminated on the moon has its own name.

The complete lunar cycle lasts 29 days.

The entire cycle is the new moon, waxing moon, waxing quarter, full moon, waning quarter, waning moon, and black moon.