double spent

The act of spending bitcoin that is no longer accessible to the sending wallet to trick the receiver into thinking they received a payment. Users most susceptible to this type of attack are those accepting 0 confirmation transactions (aka accepting a payment to be settled right away and not waiting for the network to give the transaction at least 1 transaction confirmation. Bitcoin does not solve the double spend problem but it does provide the only real-world workable solution to prevent users from falling for such attacks.