The name supermoon was coined by astrologer Richard Nolle in 1979, arbitrarily defined as... 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.
This December 3, supermoon appears full for about three days, since late Friday night through today early evening.
This full moon is the first of three consecutive supermoons. The two will occur on Jan. 1 and Jan. 31, 2018. The closest supermoon of the century will occur on December 6, 2052.