What is "Dredge" in MTG?

"Dredge" is a card mechanic in Magic: The Gathering that allows a player to cast a card from his graveyard by foregoing his normal draw and by discarding cards from the top of his deck in lieu of paying with mana. This subverts several core mechanics of the game.

The Dredge mechanic on a card is presented with a number value that indicates the number of cards the player must discard to return a card from the graveyard. For instance, the Golgari Thug has Dredge four. This allows a player to simultaneously avoid mana limitations and "mill" their deck down, a term derived from the popular Millstone card, with the assumption that many "milled" cards can themselves be Dredged.