A non-conforming mortgage is a term in the United States for a residential mortgage that does not conform to the loan purchasing guidelines set by the Federal National Mortgage Association /Federal Home Loan Mortgage Corporation (Fannie Mae and Freddie Mac).
In the context of US mortgages, a non-conforming loan refers to a home financing arrangement that doesn't conform to the standards and regulations set by Fannie Mae and Freddie Mac, the Federal National Mortgage Association/Federal Home Loan Mortgage Corporation.