Final answer:
The acceleration clause in a mortgage allows the bank to demand the full loan balance from the borrower immediately upon default.
Step-by-step explanation:
The clause that allows a bank to declare the entire mortgage balance due if the borrower defaults is typically referred to as an 'acceleration clause.' This provision enables the lender to demand full repayment of the outstanding loan balance immediately, rather than adhering to the initial payment schedule, if the borrower fails to meet the terms of the mortgage contract. Should the borrower default, this clause is often exercised to mitigate the financial risk to the lender, which can lead to foreclosure if the borrower is unable to satisfy the debt as requested.