Selling stocks that the investor does not own. That is, one can borrow stocks from brokerage houses and sell them. One can sell short if he or she believes the price will decline. Profit is determined by the price at which one borrows at, less the price at which one buys a security back at. Some short sales may involve naked positions which means the investor does not own the stocks to cover the short position. Although short selling might be a good strategy in a down market, it involves an unlimited loss if the stock price goes up instead of down.
Selling a security or commodity that you do not yet own, believing it will fall in price and can be bought later at a lower price producing a profit for you when you deliver it to the purchaser.
The sale of a borrowed security in anticipation that the price of the security will decline and that a profit can be made by buying back the security at a lower cost (and returning it to the lender) in the future.