Mill

Mill is an informal term used in the game describing cards moving out of a player's library to somewhere even less useful (meaning: anywhere other than the hand).

The term refers to the card, Millstone, which was among the first to feature the mechanic. Milling is a strategy some decks use which take advantage of the decking rule. That rule is, "If an effect would cause a player to draw more cards than remain in his or her library, that player draws the remaining cards, then loses the game the next time a player would receive priority. This is a state-based effect."

Presumably, emptying your opponent's library is one way to ensure a game loss, though you have to wait for your opponent to actually draw a card, a fine point for newcomers to the rule.