Rotation (TCG)
A "rotation" in the Pokémon Trading Card Game is when several sets are removed from Pokémon Organized Play (POP), usually because they are too old and would not be compatible with newer sets or rules. Rotations almost always occur directly following the TCG world championships and remove four or more sets. If a set is "removed," that means a player is no longer able to use any card in that set in their deck. This rule only applies to Modified Format. In Unlimited Format, players may use any card from any set in their decks.
Controversy
A fair number of players in the POP leagues believe that "banning" cards from play, and thereby removing whole decks, slows down play and is bad for the game as a whole. On the other hand, many players think that being forced to come up with new deck ideas makes the game more fun and interesting.
Coming Up
The next rotation is scheduled for directly after this years TCG world championships, and wil remove the following sets from POP:
Ex Deoxys Ex Emerald Ex Unseen Forces Ex Delta Species Ex Legend Maker.
Probably most notable of the removed cards is Holon Transciever from the Ex Delta Species set, and the whole Holon Engine with it. With the removal of Metagross and Dragonite, both from the Ex Delta Species set, the Metanite deck will no longer be playable.