What is Sum Certain?

Legal Definition
A sum certain is a specified and set amount of money owed by one person to another. It is a legal term of art, having specialized meaning in the law. Some kinds of legal claims can not be brought at all unless the sum certain can be plead. A document claimed to be a negotiable instrument can not be negotiated unless it is for a sum certain.
-- Wikipedia
Legal Definition
The settled or fixed amount that is specified in or is ascertained from negotiable instruments as a sum payable.