Most lenders would consider a conventional mortgage as a loan that conforms to the guidelines set forth by Freddie Mac and Fannie Mae, the two government sponsored enterprises (GSEs) that provide liquidity in the mortgage market.

Technically speaking, a conventional loan is any mortgage that is not guaranteed or insured by the US government, such as VA, FHA and USDA.

Conventional mortgages include portfolio loans, construction loans, and even subprime loans. But again, whenever a lender refers to a "conventional loan" they are most likely referring to conforming mortgages that are eligible for purchase by Fannie Mae and Freddie Mac.