Principal balance
The principal balance, in regard to a mortgage, loan, or other instrument of debt, is the amount due and owed to satisfy the payoff of an underlying obligation. It is distinct from, and does not include, interest or other charges. Amortized mortgage loans automatically pay a portion of each monthly payment to the principal balance, with the rest being paid as interest. An interest-only loan doesn't require any money to be paid toward the principal balance each month, but such payment is allowable. See also 
Unpaid principal balance

References

Mortgage