the lunisolar calendar used by Jews, as for determining religious holidays, that is reckoned from 3761 b.c. and was established by Hillel II in the 4th century a.d., the calendar year consisting of 353 days (defective year), 354 days (regular year), or 355 days (perfect year or abundant year) and containing 12 months: Tishri, Heshvan, Kislev, Tevet, Shevat, Adar, Nisan, Iyar, Sivan, Tammuz, Av, and Elul, with the 29-day intercalary month of Adar Sheni added after Adar seven times in every 19-year cycle in order to adjust the calendar to the solar cycle. The Jewish ecclesiastical year begins with Nisan and the civil year with Tishri. Also called Hebrew calendar. See table under calendar.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.