Below is an excerpt from an interesting article on Super Moons from The Economist
Supermoons are not particularly unusual—the next full Moon is the biggest of either two, three or four consecutive supermoons, depending on who you ask. The term “supermoon” was first used in 1979 by Richard Nolle, a peddler of astrology, to describe a Moon that is full—when it is directly opposite the Sun, as seen from Earth, and its near side is thus fully illuminated—at the same time that it reaches the closest point to Earth in its orbit. (Its technical name is a perigee-syzygy Moon—perigee referring to its closest point, and syzygy being the term for three or more astronomical bodies arranging themselves in a straight line.) Throughout its elliptical orbit, the Moon’s distance from Earth varies between about 360,000km (223,694 miles) and 400,000km. At perigee the Moon’s centre is on average around 363,300km from Earth. Not everyone agrees on exactly how close to Earth the Moon has to be in order to qualify as a supermoon. Some publications described March’s full Moon as one, but at 360,309km from Earth, it was not close enough for others. According to NASA, the strictest definitions for this year class only the full Moons in April (357,378km away from Earth) and May (closer by about 157km) as supermoons.