IOU

IOU. Greek hypothesis, a placing under; supposition. "I owe you". A popular designation of a due-bill or memorandum of debt. Consist's of those letters, a sum of money, and the debtor's signature.

IOU, contracts. The memorandum IOU, (I owe you), given by merchants to each other, is a mere evidence of the debt, and does not amount to a promissory note. Esp. Cas. N. A. 426; 4 Carr. & Payne, 324; 19 Eng. Com. L. Rep. 405; 1 Man. & Gran.

IOU: A written confirmation of a debt, signed by the debtor, which implies an undertaking to pay the sum owed at some future date. An IOU is not a negotiable instrument and may not be passed on to a third party.

A promissory note is similar to, but differs from, an IOU.
Canada's Bills of Exchange Act (Revised Statutes of Canada 1985, Chapter B-4, at §176, published at canlii.org/ca/sta/b-4/) defines a promissory note as: ...

of check known by the name of memorandum checks; these are given in general with an understanding that they are not to be presented at the bank on which they are drawn for payment; and, as between the parties, they have no other effect than an IOU or ...

See also: Promissory note, Money, Will, Parties, Time