Dance Dance Revolution X2, abbreviated DDR X2 or simply X2, is a music video game, and a part of the Dance Dance Revolution series.
Over 70 new songs are included in X2. All licensed songs from Dance Dance Revolution SuperNova and Dance Dance Revolution SuperNova 2 (With the exception of "Angelus" and "Golden Sky") have been removed due to their licenses expiring. However, 4 classic licensed songs from earlier DDR mixes have returned in remastered form, along with a special remixed license track.
One of the new features included in the main game mode is a special "extra stage" system. Known as Replicant D-action, this mode allows players to choose from a set of 6 difficult songs. When a player gets rank AA or more one of these songs (any difficulty), they receive a medal for that particular track. When players subsequently earn all 6 medals, they can access the Encore Extra Stage, "Valkyrie dimension".
Source: Wikipedia, "Dance Dance Revolution X2", available under the CC-BY-SA License.