An order given by an investor instructing his/her broker to cancel a previously placed order.

CFO's are typically followed by new orders placed on the same securities. For example, if you placed a limit order to sell Cory's Tequila Corporation (CTC) at $15 and then decided that you wanted to sell it at market, then you would CFO your first order and enter in a new order to sell CTC at market.

If you change your mind about an order, CFO's are important to enter because you don't want to be transacting your securities twice.