Dumbledore's Army (often referred to as the D.A.) is an association in the Harry Potter series by J.K. Rowling. It is started in Harry Potter and the Order of the Phoenix and continued in the last book, Harry Potter and the Deathly Hallows by member Neville Longbottom. This is an association created to prepare some pupils for beating Voldemort.
The leader, Harry Potter, teaches his friends how defend themselves against the Dark Lord. The idea for the D.A. came from Hermione Granger, who wanted the students to learn defensive spells which were not being taught by the new Defense Against the Dark Arts teacher, Delores Umbridge.