An iceberg order is a type of order placed on a public exchange. The total amount of the order is divided into a visible portion, which is reported to other market participants, and a hidden portion, which is not. When the visible part of the order is fulfilled, a new part of the hidden portion of the same size becomes visible.
As an example, suppose that a market participant places an order on the London Stock Exchange to buy 1,000 shares of stock AAAA at no more than 120p per share, with a visible portion of 100 shares. Other traders will see a buy order for 100 shares at 120p, with the other 900 remaining 'dark' or hidden. If someone places an order to sell 100 shares at 120, then the visible portion will be fulfilled. A new visible order to buy 100 at 120 will appear on the order book, and an order to buy 800 shares will remain hidden.
See also: http://www.investopedia.com/terms/i/icebergorder.asp