It stands for all or none; an order (trading order) that is placed along with a stipulation that it cannot be filled or executed unless with a full fill (not a partial fill). If not filled in its entirety, the order remains open (an open order). This order comes with a restriction as to not fill “partially”. This tells the market makers in a given market to not execute the order unless the entire number of contracts can be filled.

Due to the underlying restriction, all or none orders are not regular orders, and hence will not be stored in the regular lot book of an exchange (which typically contains regular lot orders, only). These orders are rather stored in the so-called special terms book.