nominal-principal-only (NPO): 1. One of the secondary payment streams, which is included in the real-principal-only payment stream. 2. The payment stream representing the par value, or the nominal amount paid for the instrument upon its original issuance. With bonds, such as the TIPS, the nominal-principal-only is paid when the bond reached maturity. 3. Part of the accrued principal, along with the inflationary adjustment (or AIO payment stream).