What is "mirrodin"

Mirrodin is the name of the Magic: The Gathering expert-level block containing the Mirrodin (October 2, 2003, 306 cards), Darksteel (February 6, 2004, 165 cards) and Fifth Dawn (June 4, 2004, 165 cards) expansion sets. This expansion, as well as the rest of the block, is centered on artifacts and was only the second set to do so (from a card frequency point of view) since Antiquities. The percentage of artifact cards is much higher than in any preceding set. Each of the expansion symbols in the Mirrodin block depicts a legendary artifact: (Mirrodin), (Darksteel) and (Fifth Dawn).