A short sale is the sale of a security that the seller does not own.
What is a short sale?
This is a highly speculative form of trading where the seller believes that the price of a security is going to fall and they will be able to cover the sale by buying back the security at a lower price.
The profit would be the difference between the initial selling price and the subsequent purchase price. It is an illegal practice not to declare the trade a "short sale" when the order is placed.