The Collaborative International Dictionary of English v.0.48:

Passover \Pass"o`ver\, n. [Pass + over. See Pasch.] (Jewish
   Antiq.)
   (a) A feast of the Jews, instituted to commemorate the
       sparing of the Hebrews in Egypt, when God, smiting the
       firstborn of the Egyptians, passed over the houses of the
       Israelites which were marked with the blood of a lamb.
   (b) The sacrifice offered at the feast of the passover; the
       paschal lamb. --Ex. xii.
       [1913 Webster]

WordNet (r) 3.0 (2006):

Passover
    n 1: (Judaism) a Jewish festival (traditionally 8 days from
         Nissan 15) celebrating the exodus of the Israelites from
         Egypt [syn: Passover, Pesach, Pesah, Feast of the
         Unleavened Bread]