"When the fourth cup is poured, over which Hallel is recited, it is customary to pour an additional cup, one bigger than the others. This fifth cup is called the Cup of Eliyahu. Its basis is a Talmudic debate (Pesachim 118a): Rabbi Tarfon maintained that one must drink five cups of wine at the Seder, the fifth being the one Hallel is recited over. Since the dispute is unresolved, the custom is to pour the fifth cup – as per Rabbi Tarfon – but not to drink it – as per the Sages. When Eliyahu comes and clarifies all of our halachic doubts, this too shall be resolved. That is why it is called the Cup of Eliyahu."

haggadah Section: Nirtzah
Source: http://www.morashasyllabus.com/class/The%20Passover%20Seder.pdf