A double jump shift shows support for partner's suit and a singleton or void in the suit named
South North 1H 4D
4D shows a singleton or void diamond and suggests strong support for hearts.
A bidding convention is an agreement between two players on a different meaning of a bid in the card game bridge. These agreements are made so that partners can more quickly determine which contract, with both their cards, they can win together. Often, conventions are forged together into one, which is then called a bidding system.