The definition of American subprime mortgage crisis is the subprime mortgage crisis in American real estate market. Subprime mortgage is relative to mortgage with good credit conditions. Mortgage lenders have no (or lack of) proof of income/repayment ability, or other liabilities are heavy, so their credit conditions are "sub-optimal". This kind of real estate mortgage is called subprime mortgage.

Compared with the more favorable interest rates and repayment methods available to mortgage lenders with better credit conditions, subprime mortgage lenders are usually forced to pay higher interest rates and abide by stricter repayment methods. This was originally a natural problem, but it has not been really implemented because of the influence of loose credit, active financial innovation and rising prices of real estate and securities markets in the United States in the past six or seven years. In this way, the repayment risk of subprime mortgage will turn from potential to reality. In this process, some financial institutions in the United States connived at the excessive expansion of subprime mortgage and its related loan packaging and bond scale for their own interests, which made the scale of subprime mortgage default events expand to the extent of causing crisis under certain conditions.