Definition of "supermoon"

And should the moon happen to hit its ever-shifting orbital perigee at the same time that it lies athwart from the sun, we are treated to a so-called supermoon, a full moon that can seem close enough to embrace – as much as 12 percent bigger and 30 percent brighter than the average full moon. […] Some astronomers dislike the whole supermoon hoopla. They point out that the term originated with astrology, not astronomy; that perigee full moons are not all that rare, coming an average of every 13 months; and that their apparently swollen dimensions are often as much a matter of optical illusion and wishful blinking as of relative lunar nearness.

2014 September 7, Natalie Angier, “The Moon comes around again ”, in The New York Times‎