An agreement in which the seller of the property agrees to provide a portion of the financing. The buyer will then essentially have two mortgages, one which they pay to the bank monthly, and the other which they pay to the seller of the property monthly.

Example

A buyer would like to buy a $200,000 house from a seller, however the bank will only mortgage the buyer $180,000. The seller can then choose to "carry back" the remaining $20,000, and essentially mortgage it to the buyer. The buyer then has 2 mortgages; one for $200,000 to the bank, and one for $20,000 to the seller.