Looking For Short Sale Definition

My wife and i have been having a great deal of trouble keeping up with our mortgage payment.  It has always been a little too high for us to be comfortable with it, but when my wife lost her job it became almost impossible.  I am really afraid we are going to end up losing the house.  I would like to avoid foreclosure if at all possible and am trying to find the short sale definition.  I have read about this and want to find out a little more about this in order to determine if this is an option for us.