Concerning the lunar month of approximately 29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon‘s directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the terminology of the four major phases (new moon, first quarter, full moon, last quarter) and four minor phases (waxing crescent, waxing gibbous, waning gibbous, and waning crescent)

Full moon is always opposite to sun

the sun

A full moon is opposite the sun in its orbit around Earth. Its sunlit side is entirely visible from Earth. The moon appears full to the eye for two to three nights. However, astronomers regard the moon as full at a precisely defined instant, when the moon is exactly 180 degrees opposite the sun in ecliptic longitude