Surge

Surge is a keyword ability introduced in Oath of the Gatewatch that provides an alternative cost that if the spell is set up with another spell.

Description
You may cast a spell for its surge cost if you or a teammate have cast another spell in the same turn. However, the triggered ability only gives a bonus to your own creatures, not to your teammate's. The cards are both cheaper and more powerful if cast for their surge cost.

Teammates are all allied players in Two-Headed Giant, Emperor, and certain other multiplayer Magic formats. The development team balanced the mechanic such that the cards could be powerful in two-player games as well.

Rulings

 * For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.
 * The other spell that you or a teammate cast can be one that's resolved, one that was countered, or (for instants with surge) one that's still on the stack.
 * If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.
 * Casting a spell for its surge cost doesn't change its mana cost or its converted mana cost.