A jumbo loan is a mortgage whose principal balance exceeds the maximum Fannie Mae or Freddie Mac may buy, and which therefore has to be held by the lender or sold into the private market rather than to a government-sponsored enterprise. For a one-unit property in most of the country that threshold is $832,750, but it is higher in high-cost counties, higher again for multi-unit properties, and higher still in Alaska, Hawaii, Guam and the U.S. Virgin Islands, so the same loan amount can be jumbo in one place and ordinary in another.
The naming deserves a precise answer, because the usual one is vague. "Jumbo" is a market word, and the closest federal law comes to adopting it is instructive: it appears exactly once in Regulation Z, in quotation marks, in the Official Interpretations. Comment 35(a)(1)-3, headed "Threshold for 'jumbo' loans", explains that Regulation Z "provides a separate threshold for determining whether a transaction is a higher-priced mortgage loan … when the principal balance exceeds the limit in effect as of the date the transaction's rate is set for the maximum principal obligation eligible for purchase by Freddie Mac (a 'jumbo' loan)." So the regulator uses the word as shorthand while declining to define it, and the actual legal test is stated by reference to the Freddie Mac limit rather than to any independent standard.