The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

September
    n 1: the month following August and preceding October [syn:
         September, Sep, Sept]