Current usage or "short sale" in the real estate world is when the mortgagee agrees to release its claim for less than full payment. I'm not in the lending business, but I assume the lender weighs the cost/benefit of taking what it can get in today's market vs foreclosing.
All lienholders have to agree, but since everyone but the first mortgage would be wiped out in a foreclosure, lesser claims really don't have much reason not to agree, assuming the property is going for FMV.