Sunk cost is a term in accountancy. It refers to monies which have already been paid and are unrecoverable. It is an important principle of rational decision-making that sunk costs must be ignored in decision-making. For example, suppose $1 million has already been spent on a project that is being being considered for cancellation. If the project is stopped or abandoned that money will have been wasted. However, that fact should not be considered when deciding whether or not to stop the project, as that money is already gone.