A Kill is a request to cancel a trade between its placement and its fulfillment. In other words, when you place an order to buy or sell a security, you may "kill" the order before it is executed if you no longer wish to trade at that price.This can be useful if the market moves suddenly, and you don't want to be stuck with a losing trade. Of course, if the market moves in your favor after you've placed your kill order, your trade will never be executed, and you'll miss out on potential profits.