Douglas Harper's Etymology Dictionary
1856, from Rabbinical Hebrew haggadhah, literally "tale," verbal noun from higgidh "to make clear, narrate, expound."
Wiktionary
n. 1 (context Judaism English) A text that sets forth the order of the Passover seder. 2 (alternative form of Aggadah English)
Wikipedia
The Haggadah (, "telling"; plural: Haggadot) is a Jewish text that sets forth the order of the Passover Seder. Reading the Haggadah at the Seder table is a fulfillment of the Scriptural commandment to each Jew to "tell your son" of the Jewish liberation from slavery in Egypt as described in the Book of Exodus in the Torah ("And thou shalt tell thy son in that day, saying: It is because of that which the did for me when I came forth out of Egypt." ).
Sephardi and Mizrahi Jews also apply the term Haggadah to the service itself, as it constitutes the act of "telling your son."
Usage examples of "haggadah".
The table had been set with a sparkling white tablecloth and Passover dishes, matzah and bitter herbs and Avruhm read from the Haggadah in Hebrew.
This part had always been a little funny to Scott because the Haggadah directs that, as each plague is named, those at the table are to stick a finger in their wine and sprinkle a drop on their plate.
It is included in the Haggadah, the book that Jews read aloud while conducting the seder, the festive meal they are required to eat on the holiday of Passover.