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]

Easton's 1897 Bible Dictionary:

Passover
   the name given to the chief of the three great historical annual
   festivals of the Jews. It was kept in remembrance of the Lord's
   passing over the houses of the Israelites (Ex. 12:13) when the
   first born of all the Egyptians were destroyed. It is called
   also the "feast of unleavened bread" (Ex. 23:15; Mark 14:1; Acts
   12:3), because during its celebration no leavened bread was to
   be eaten or even kept in the household (Ex. 12:15). The word
   afterwards came to denote the lamb that was slain at the feast
   (Mark 14:12-14; 1 Cor. 5:7).

     A detailed account of the institution of this feast is given
   in Ex. 12 and 13. It was afterwards incorporated in the
   ceremonial law (Lev. 23:4-8) as one of the great festivals of
   the nation. In after times many changes seem to have taken place
   as to the mode of its celebration as compared with its first
   celebration (comp. Deut. 16:2, 5, 6; 2 Chr. 30:16; Lev.
   23:10-14; Num. 9:10, 11; 28:16-24). Again, the use of wine (Luke
   22:17, 20), of sauce with the bitter herbs (John 13:26), and the
   service of praise were introduced.

     There is recorded only one celebration of this feast between
   the Exodus and the entrance into Canaan, namely, that mentioned
   in Num. 9:5. (See JOSIAH.) It was primarily a
   commemorative ordinance, reminding the children of Israel of
   their deliverance out of Egypt; but it was, no doubt, also a
   type of the great deliverance wrought by the Messiah for all his
   people from the doom of death on account of sin, and from the
   bondage of sin itself, a worse than Egyptian bondage (1 Cor.
   5:7; John 1:29; 19:32-36; 1 Pet. 1:19; Gal. 4:4, 5). The
   appearance of Jerusalem on the occasion of the Passover in the
   time of our Lord is thus fittingly described: "The city itself
   and the neighbourhood became more and more crowded as the feast
   approached, the narrow streets and dark arched bazaars showing
   the same throng of men of all nations as when Jesus had first
   visited Jerusalem as a boy. Even the temple offered a strange
   sight at this season, for in parts of the outer courts a wide
   space was covered with pens for sheep, goats, and cattle to be
   used for offerings. Sellers shouted the merits of their beasts,
   sheep bleated, oxen lowed. Sellers of doves also had a place set
   apart for them. Potters offered a choice from huge stacks of
   clay dishes and ovens for roasting and eating the Passover lamb.
   Booths for wine, oil, salt, and all else needed for sacrifices
   invited customers. Persons going to and from the city shortened
   their journey by crossing the temple grounds, often carrying
   burdens...Stalls to change foreign money into the shekel of the
   temple, which alone could be paid to the priests, were numerous,
   the whole confusion making the sanctuary like a noisy market"
   (Geikie's Life of Christ).