What does the title of the World of Warcraft expansion refer to?

“The Burning Crusade” refers to the Burning Legion’s ongoing efforts to destroy life on Azeroth. To date, the Burning Legion’s exploits have been documented in Warcraft III and in our War of the Ancients novel trilogy. As part of World of Warcraft: The Burning Crusade, players will now be able to travel through the Dark Portal to confront the Legion on otherworldly battlefields.