When-issued trading is trading in securities that have not yet been issued. The most common type of when-issued trading involves Treasury securities.
The week before it sells new bills, notes or bonds at auction, the Treasury Department announces the size of the new issue or issues. At that point, people start trading them on a when-issued basis. As in a normal transaction, a buyer and a seller agree on a price. But the exchange of cash and securities is delayed till the securities are issued.
Security that are traded on a when-issued basis are termed when-issued, or w.i., securities.