Sale and purchase of a stock within a very short time frame, usually to manipulate taxes, where a gain can be washed out by losses on the transaction.
Law Dictionary – Alternative Legal Definition
In the language of the stock exchange, this is the operation performed by a broker who fills an order from one customer to buy a certain stock or commodity by simply transferring to him the stock or commodity placed in his hands (or ordered to be sold) by another customer, instead of going upon the exchange and executing both buying and selling orders separately and on the best terms obtainable for the respective customers.