What is Principal Balance?

Legal Definition
The principal balance, in regard to a mortgage or other debt instrument, is the amount due and owing to satisfy the payoff of the underlying obligation, less 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 does not require any money to be paid toward the principal balance each month, but such payment is allowable.
-- Wikipedia
Legal Definition
The loan amount that has yet to paid. It is not inclusive of interest and other related charges.