The term 'moon stage' refers to the different phases of the moon. The most common PHASE is the five-letter word 'PHASE', which describes the various stages of the moon's cycle, from new moon to full moon. The moon's cycle is caused by the changing positions of the sun, earth, and moon relative to one another. During a full moon, the moon is completely illuminated by the sun, while during a new moon, the moon is not visible from Earth.