What term is used to describe the phenomenon of having two full moons in a single calendar month?

The term that describes having two full moons in a single calendar month is "blue moon." This occurrence typically happens because the lunar cycle is about 29.5 days long, yet many months have 30 or 31 days. Therefore, it is occasionally possible for a month to fit two full moons within that timeline. A blue moon can be defined as the second full moon that falls in a single calendar month. This characteristic distinguishes it from other moon-related terms.

The other terms refer to different celestial phenomena: "supermoon" describes a full moon that appears larger and brighter due to its proximity to Earth, while "harvest moon" refers specifically to the full moon that occurs closest to the autumnal equinox. A "new moon" is the phase in which the moon is not visible from Earth as it is located between the Earth and the sun. Each of these terms has its own unique definition and significance that does not relate to the concept of dual full moons within one month.