Easton's Bible Dictionary
The name adopted from the Babylonians by the Jews after the Captivity for the third civil, or ninth ecclesiastical, month (Nehemiah 1:1; Zechariah 7:1). It corresponds nearly with the moon in November.
Noah Webster's New International Dictionary of the English Language
(n.) The ninth month of the Jewish ecclesiastical year, answering to a part of November with a part of December.