The moon is called a "full moon" when it is fully illuminated and appears to be round from the perspective of the earth. A full moon occurs when the moon is on the opposite side of the earth from the sun.
Due to the moon's orbit and the relative positions of the earth, moon and sun, different parts of the moon's surface are lit by the sun at different times, which creates the various phases of the moon. Time intervals between similar phases average approximately 29 days and occur in a predictable pattern. The part of the moon that appears round during a full moon is always the same side of the moon's surface because the moon rotates only once each month.