From WordNet (r) 2.0 (August 2003) [wn]:
September
n : the month following August and preceding October [syn:
September, Sep, Sept]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]