What does the term 'bank guarantee' mean?

A bank guarantee is a commercial instrument in the nature of a contract, intended between two parties, to secure compliance with the contract. It is an off-shoot of the main contract between two parties. A bank gaurantee is a guarantee made by a bank on behalf of a customer (usually an established corporate customer) should it fail to deliver the payment, essentially making the bank a co-signer for one of its customer's purchases.