The Collaborative International Dictionary of English v.0.48:

Adar \A"dar\, n. [Heb. ad[aum]r.]
   The twelfth month of the Hebrew ecclesiastical year, and the
   sixth of the civil. It corresponded nearly with March.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Adar
    n 1: the sixth month of the civil year; the twelfth month of the
         ecclesiastic year in the Jewish calendar (in February and
         March)

Easton's 1897 Bible Dictionary:

Adar
   large, the sixth month of the civil and the twelfth of the
   ecclesiastical year of the Jews (Esther 3:7, 13; 8:12; 9:1, 15,
   17, 19, 21). It included the days extending from the new moon of
   our March to the new moon of April. The name was first used
   after the Captivity. When the season was backward, and the lambs
   not yet of a paschal size, or the barley not forward enough for
   abib, then a month called Veadar, i.e., a second Adar, was
   intercalated.


Hitchcock's Bible Names Dictionary (late 1800's):

Adar, high; eminent