Unsecured debt (IOU), issued by large corporations, with maturities (at time of issue) less than a year. They can be traded on OTC.
Borrowing in the form of short term paper issued by companies. Also known as CP or ECP (Euro Commercial Paper). Usually redeemable at par value and pays no interest. Hence it is issued and trades at a discount.
Commercial Paper (CP) is short term unsecured debt issued by companies in the form of promissory notes as an obligation of the issuer. CP is typically issued at a discount to face value - but interest bearing notes can be requested. If paper is issued as interest bearing it will still be quoted on a discount basis. CP can be issued in bearer or registered form. The scale of any CP issue makes it exclusively a wholesale market, attracting banks, money market funds, insurance companies and other large cash rich firms as investors.