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Fast effects is an obsolete collective term for instants, interrupts and most special powers of cards in play, including the tapping of land for mana. Fast effects were called that because they could start before another spell was finished.
In the First Edition, the rules said that fast effects could be used "at any time". This turned out to cause a number of problems and paradoxes, so the Revised rules have removed that definition and instead specified when fast effects could be used. The term was retired with the Sixth Edition rules change