A double, intended for take-out, used by the player in the pass-out seat to keep the auction alive. A common situation is for opener to reopen with a double as an effective way of competing after partner has passed over an opponent’s intervention, especially if negative doubles are part of the system.
Double is a much better call than 2♦ partly because it gives East the option to convert to penalties by passing and partly because the best contract may be in hearts.