Mill

From MTG Salvation Wiki

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 the first to feature the mechanic. "Milling" is a strategy some decks use which take advantage of the decking rule:

From the Comprehensive Rules 102. Winning and Losing 102.3. There are several ways to lose the game. 102.3c When a player is required to draw more cards than are left in his or her library, he or she draws the remaining cards, and then loses the game the next time a player would receive priority. (This is a state-based effect. See rule 420.)

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 to the rule.