Reaction is a card type. Reaction cards have blue frames, and appear in most expansions. "Reaction" is always a subtype, and the Reaction effect of a card is almost always detailed under a dividing line. As the name would suggest, Reactions have their effects triggered in response to something else happening; when its particular trigger occurs, a Reaction may be revealed by the player in order to get its effect. These triggers usually happen when the card is in its owner's hand or otherwise not visible to opponents; cards with when-gain or when-trash abilities are not considered to be Reactions.
Except for Caravan Guard, the Reaction effects of cards do not involve playing them, only revealing or discarding or trashing them. Reaction effects can occur anytime their trigger happens, not just during your own turn.
 List of Reaction Cards
There are no official rules for Reaction cards in general. How to use them is detailed in the FAQ for each individual card, and in the rules descriptions of the specific mechanics on those cards.
Some Reaction cards' abilities instruct a player to discard them, trash them, play them, or set them aside when they are revealed for their Reaction effects. In the absence of such instructions, after being revealed, the card returns to wherever it was revealed from before the rest of its abilities are resolved. For instance, if one player plays an Attack card and another reveals a Moat as a Reaction, the Moat returns to the second player's hand and may be revealed again against future Attacks, and may be played (as an Action) on their next turn. In such cases, it is even permissible to reveal the same Reaction multiple times in response to the same event; however, Reaction abilities are balanced in such a way that doing so rarely has any additional effect.
It is permissible to reveal a Reaction from one's hand in response to an event even if the Reaction was not yet in hand when the triggering event took place, provided that the ability that puts the Reaction in hand was also triggered by the same event that triggered the Reaction. For instance, if an opponent plays an Attack, you may reveal a Secret Chamber and draw a Moat and a Beggar; if the Moat and Beggar are still in hand after the Secret Chamber is resolved, you may reveal the Moat and Beggar in reaction to the same Attack that triggered the Secret Chamber.
Many Reaction cards may be revealed in response to an opponent playing an Attack card, and offer some type of defense against some or all Attack cards. Several Reactions that react to events other than Attacks may also offer some protection from Attacks; for instance, Watchtower's Reaction ability protects a player against junking attacks, and its Action ability protects a player against handsize attacks. Since the first Reaction card that many players encounter is Moat, it is a common misconception among new players that all Reactions are revealed in response to Attacks being played, or even that all Reactions offer Moat-like cancellation of Attack effects. However, not all Reactions offer any protection against or interaction with Attacks at all (e.g., Fool's Gold), and not all cards that protect a player against attacks are Reactions (e.g., Lighthouse).
 In other languages
- Czech: Reakce
- Dutch: Reactie
- Finnish: Vastaveto (lit. counter-move)
- German: Reaktion
- Polish: Reakcja
- Russian: Реакция (pron. ryeaktsiya)