An acceleration clause is a provision clearly stated in a contract (mortgage) that grants a lender (mortgagee) the right to immediately recall a loan or demand full repayment of the outstanding loan balance amount owed.
It can be triggered upon an act that the lender deems as a breach of contract, like a default.
For example when a property is sold by the owner, defaults on installment repayments, fraudulent documentation, false credit declarations, property value depreciates to less than amount owing, etc