Source : Webster's Revised Unabridged Dictionary (1913)

September \Sep*tem"ber\, n. [L., fr. septem seven, as being the
   seventh month of the Roman year, which began with March: cf.
   F. septembre. See {Seven}.]
   The ninth month of the year, containing thurty days.

Source : WordNet®

September
     n : the month following August and preceding October [syn: {Sep},
          {Sept}]