An acceleration clause is a provision in a mortgage or loan contract that allows the lender to demand the full repayment of the loan if certain conditions are not met. For example, if the borrower misses a payment or violates the terms of the loan, the lender can invoke the acceleration clause and demand the full repayment of the loan. This clause gives the lender more power to protect their investment and ensures that the loan will be repaid in a timely manner.