Madness

Madness is a Keyword Ability that allows you to pay an alternate cost for a spell when that spell is discarded. It was first introduced in the Odyssey Block in Torment.

From the Comprehensive Rules:

• 502.24. Madness
• 502.24a Madness is a keyword that represents two abilities. The first is a static ability that functions while the card with madness is in a player’s hand. The second is a triggered ability that functions when the first ability is applied. "Madness [cost]" means "If a player would discard this card, that player discards it, but may remove it from the game instead of putting it into his or her graveyard" and “When this card is removed from the game this way, until that player passes next, the player may play it any time he or she could play an instant by paying [cost] rather than paying its mana cost. When the player passes next, he or she puts this card into his or her graveyard."
• 502.24b Playing a spell using its madness ability follows the rules for paying alternative costs in rules 409.1b and 409.1f–h.

Madness inspired a deck known as "Wild Mongrel", which, using the signature card Wild Mongrel, enabled instant-speed discard, which allowed cards with Madness to be played at instant-speed. Wild Mongrel also enabled other abilities such as Flashback and the seven-card cycle known as "The Incarnations". (Anger, Brawn, Filth, Genesis, Glory, Valor and Wonder.)