Megillah is the tenth Tractate of Mishnah in the Order Moed. It and its Gemara deal with the laws of Purim and offers exegetical understandings to the Book of Esther. It also includes laws concerning the public reading of the Torah and other communal synagogue practices. There is also a segment in the first chapter which details certain miscellaneous laws.
(A megillah is a finely detailed account or book but the term by itself commonly refers to the Book of Esther.)