WANE (5 letters) The term 'moon phase' refers to the different stages of the moon's cycle, which can be observed from Earth. The most well-known moon phase is the FULL moon, which is when the moon is completely illuminated and appears as a bright circle in the night sky. The opposite of a FULL moon is a waning moon, which is when the moon is gradually decreasing in illumination and appears as a crescent shape in the night sky.