Sell market order
A sell market order sells immediately into the best bids in the order book. It guarantees the whole exit or entry at once, but not the price.
It hits the highest bid first, then the next bids down if needed, so in a thin book or a fast fall you get less than the price you saw. It is how a stop loss on a long finally sells. The opposite is a buy market order.
Learn more: types of orders.