The Collaborative International Dictionary of English v.0.48:

Purim \Pu"rim\, n. [Heb. p[=u]r, pl. p[=u]r[imac]m, a lot.]
   A Jewish festival, called also the Feast of Lots, instituted
   to commemorate the deliverance of the Jews from the
   machinations of Haman. --Esther ix. 26.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Purim
    n 1: (Judaism) a Jewish holy day commemorating their deliverance
         from massacre by Haman