What is Endowment Mortgage?

Legal Definition
An endowment mortgage is a mortgage loan arranged on an interest-only basis where the capital is intended to be repaid by one or more (usually Low-Cost) endowment policies. The phrase endowment mortgage is used mainly in the United Kingdom by lenders and consumers to refer to this arrangement and is not a legal term.

The borrower has two separate agreements. One with the lender for the mortgage and one with the insurer for the endowment policy. The arrangements are distinct and the borrower can change either arrangement if they wish. In the past the endowment policy was often taken as an additional security by the lender. That is, the lender applied a legal device to ensure the proceeds of the endowment were made payable to them rather than the borrower; typically the policy is assigned to the lender. This practice is uncommon now.
-- Wikipedia
Legal Definition
Loan payment split where interest only is paid to the lender and principal repayment installments are paid into a life insurance policy. At maturity, the mortgage becomes due and is paid by the insurance payout. Contrast with repayment mortgage.