A Jumbo mortgage loan is any mortgage loan that is not underwritten to the loan limits of the government agencies of Fannie Mae, Freddie Mac, or the FHA. This is set by the lending limits in place, county by county, across the country. These limits and adjustments are based on the median home value in each county and the number of units attached to the property (up to 4) that is looking for financing.
Jumbo mortgages are typically funded, sold to, and/or serviced by portfolio lenders that will use many of the basic underwriting guidelines set forth by the government agencies, but will adjust their appetite for certain types of loans in relation to loan amount, loan to value, and credit rating (for example). They will typically carve out their own niche segment of the market that they are targeting.