Definition
Selling a home for less than the mortgage balance, with the lender's approval, because the owner is underwater and cannot keep paying. The lender agrees to accept the sale proceeds and either forgives or negotiates the shortfall. (Shorting a stock is an unrelated concept: a bet that a share price will fall.)
Why it matters
For an underwater owner in trouble, a short sale usually damages credit less than a foreclosure and ends the debt on clearer terms. The catch is lender approval, which can take months, and possible tax on any forgiven balance.