What is Bill Single?

Legal Definition
A written promise to pay to a person or persons named a stated sum at a stated time, without any condition. When under seal, as is usually the case, it is sometimes called a "bili obligatory," (q. v.) It differs from a "bili penal," (g. v.,) in that it expresses no penalty. 8. In commercial law. A written statement of the terms of a contract, or specification of the items of a transaction or of a demand; also a general name for any item of indebtedness, whether receivable or payable. -Bill-book. In mercantile law. A book in which an account of bills of exchange and promissory notes, whether payable or receivable, is stated.
-- Black's Law Dictionary