From 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]
From WordNet (r) 3.0 (2006) :
September
n 1: the month following August and preceding October [syn:
September, Sep, Sept]