The cup of Elijah is a term designating the cup of wine which is placed on the table of the Passover eve ceremonial (seder), but which is not drunk.
There was controversy among the rabbis whether the seder ritual required four or five cups. Since, according to traditional belief, all doubtful cases of tradition will be resolved "when Elijah comes," custom decreed that the fifth cup should be filled but not partaken of (cf. Pes. 118a; Maim., Yad, Ḥameẓu-Maẓẓah 8:10). Later this custom became associated with the belief that Elijah had not died but had ascended to heaven in a fiery chariot (II Kings 2:11), and that he would return as the forerunner of the Messiah (Mal. 3:23). The festival of redemption from Egyptian bondage was naturally associated with the forerunner of the Messiah, who was expected in this "season of redemption" to herald the coming deliverance (cf. RH 11b). Hence the popular notion arose that the "cup of Elijah" was prepared to welcome the prophet who visited every Jewish home on the Passover night.
Sources: Encyclopaedia Judaica. © 2008 The Gale Group. All Rights Reserved.