Definition of Short Sale (courtesy of investopedia):
Definition of 'Short Sale' A market transaction in which an investor sells borrowed securities in anticipation of a price decline and is required to return an equal number of shares at some point in the future.
The payoff to selling short is the opposite of a long position. A short seller will make money if the stock goes down in price, while a long position makes money when the stock goes up. The profit that the investor receives is equal to the value of the sold borrowed shares less the cost of repurchasing the borrowed shares.