Account stated is a statement between a creditor (the person to whom money is owed) and a debtor (the person who owes) based upon a series of prior transactions that a particular amount is owed to the creditor as of a certain date. Often the account stated is a bill, invoice or a summary of invoices, signed by the customer or sent to the customer who pays part or all of it without protest. Account stated may also be called statement of account.
A settlement, on the other hand, is a formal agreement or decision that ends an argument or dispute; or it can also refer to an amount of money that someone receives as part of such an agreement, or to the act of paying back money that is owed.
