The basic amount invested or borrowed, not including any interest or dividends.

For bonds: The face or par value of a bond. The principal amount of a trade is the par value of one bond times the number of bonds involved in the trade.

For MMS:

This category contains a return payment of an original investment in a debt product. Examples are principal payments from an amortised security such as a FNMA pools and bond partial calls, full calls and redemptions.