What is Interest-only Loan?

Legal Definition
An interest-only loan is a loan in which, for a set term, the borrower pays only the interest on the principal balance, with the principal balance unchanged. At the end of the interest-only term the borrower may enter an interest-only mortgage, pay the principal, or (with some lenders) convert the loan to a principal and interest payment (or amortized) loan at his/her option.
-- Wikipedia