From the Comprehensive Rules 4.0:
A special ability or event labeled with the word “Response:” indicates that you may perform that action whenever the trigger described in its game text happens.
You may respond more than once to the same situation.
A response action is not a phase action (because there is no “response phase”).
Some responses are performed when a described situation is “about to” happen. Typically, only one such response can be performed in a given situation, because its effect will “prevent” that situation or replace it with another effect “instead.”
See also action procedure, cost, effect.
|General||At Random||Can/Cannot||Cost||Effect||May||Limit||Pass||Prevent||Spot||Up To|
|Triggers||At the Start||Each Time||When||While|
|Phase Actions||Assign||Cancel||Special Ability||Response|
|Site Actions||At a Site||Control||Liberate||Replace|