A supermoon is a full moon or a new moon that approximately coincides with the closest distance that the Moon reaches to Earth in its elliptic orbit, resulting in a slightly larger-than-usual apparent size of the lunar disk as seen from Earth. The technical name is the perigee syzygy of the Earth-Moon-Sun system or more simply full (or new) Moon at perigee.[a] The term supermoon is astrological in origin, and has no precise astronomical definition.
The real association of the Moon with both oceanic and crustal tides has led to claims that the supermoon phenomenon may be associated with increased risk of events like earthquakes and volcanic eruptions, but no such link has been found.
Of the possible 12 or 13 full (or new) moons each year, usually three or four may be classified as supermoons, as commonly defined.
The most recent supermoon occurred on January 31, 2018, and the next (full) one will be on December 22, 2018. The one on November 14, 2016 was the closest supermoon since January 26, 1948, and will not be surpassed until November 25, 2034. The closest supermoon of the 21st century will occur on December 6, 2052.
The oscillating nature of the distance to the full or new moon is due to the difference between the synodic and anomalistic months. The interval period between full or new supermoons is roughly 14 synodic months.
A full moon at perigee appears roughly 14% larger in diameter than at apogee. Many observers insist that the moon looks bigger to them. This is likely due to observations shortly after sunset when Moon is near the horizon and the moon illusion is at its most apparent.
While the moon's surface luminance remains the same, because it is closer to the earth the illuminance is about 30% brighter than at its farthest point, or apogee. This is due to the inverse square law of light which changes the amount of light received on earth in inverse proportion to the distance from the moon. While a typical summer full moon at temperate latitudes provides only about 0.05-0.1 lux, a supermoon directly overhead in the tropics could provide up to 0.36 lux.
... a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.-- Richard Nolle
Nolle told Atlantic magazine that he created the term because "'perigee syzygy' is something that the average sixth-grader can't handle, and that's the average reading capability of the American adult," though literacy tests in the United States consistently place the mean level at 9th grade or higher.
He came up with the name while reading "Strategic Role Of Perigean Spring Tides in Nautical History and Coastal Flooding" published in 1976 by NOAA Hydrologist Fergus Wood. Nolle never outlined why he chose 90%, but explained in 2011 that he based calculations on 90% of the difference lunar apsis extremes for the solar year. In other words, a full or new moon is considered a supermoon if where is the lunar distance at syzygy, is the lunar distance at the apogee, and is the lunar distance at the perigee, and is the lunar distance at the apogee. 
In practice, there is no official or even consistent definition of how near perigee the full Moon must occur to receive the supermoon label, and new moons rarely receive a supermoon label. Sky and Telescope magazine refers to full Moon which comes within 223,000 miles (359,000 km), TimeandDate.com prefers a definition of 360,000 kilometres (220,000 mi). EarthSky uses Nolle's definition comparing their calculations to tables published by Nolle in 2000.
The term perigee-syzygy or perigee full/new moon is preferred in the scientific community. Perigee is the point at which the Moon is closest in its orbit to the Earth, and syzygy is when the Earth, the Moon and the Sun are aligned, which happens at every full or new moon. Astrophysicist Fred Espenak uses Nolle's definition but preferring the label of full Moon at perigee. Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy.
Wood also coined the less used term proxigee where perigee and the full or new moon are separated by 10 hours or less.
Nolle also claimed that the moon causes "geophysical stress" during the time of a supermoon. There has also been media speculation that natural disasters, such as the 2011 T?hoku earthquake and tsunami and the 2004 Indian Ocean earthquake and tsunami, are causally linked with the 1-2 week period surrounding a supermoon. A large, 7.5 magnitude earthquake centred 15 km north-east of Culverden, New Zealand at 00:03 NZDT on November 14, 2016, also coincided with a supermoon. These claims have been refuted by scientists.
Scientists have confirmed that the combined effect of the Sun and Moon on the Earth's oceans, the tide, is greatest when the Moon is either new or full. and that during lunar perigee, the tidal force is somewhat stronger, resulting in perigean spring tides. However, even at its most powerful, this force is still relatively weak, causing tidal differences of inches at most.
Media related to Supermoon at Wikimedia Commons