Instant

From Magic: The Gathering

Instants are cards that can be played whenever, provided you have enough mana to pay for them.

[edit] Stacking

Instants use the Stack just like every other spell, and you can respond to any spell with an instant (provided a spell with Split Second isn't on the stack).

[edit] Rules

Even though you may play instants at any phase in any turn, there are a few rules that must be observed. First, you have to have enough untapped mana in order to pay for the instant. Secondly, you must have priority.

[edit] Example

Timmy attacks with his Grizzly Bears. You have no creatures to block with, but you do have one Mountain untapped and a Shock in hand. After Timmy finishes declaring attackers, you may play the Shock targeting the Grizzly Bears. The Shock would deal 2 damage to the bears, killing them before they could deal any combat damage.