A situation where a seller of some asset (shares, bonds, commodities, etc) fails to deliver on the agreed date. This sometimes arises if the seller is selling short. The buyer, in turn, is entitled to buy the respective asset from other sellers and to charge the unfulfilling seller with any costs incurred.

Buying-in also refers to the purchase of more than 50% of a company’s shares so that the buyer can gain financial and managerial control.