A seasoning added to a limit order instructing the broker to get all the number of desired shares at a certain price. For instance if the stock is trading at $1.55 a share and the investor instructs the broker to acquire 1 million shares of that stock at that price. If all or none is not commanded by the investor to the broker, the broker will get any number of shares available at that price and wait until it can acquire all 1 million shares at that price.
A more graphic example, if there are only 500,000 shares available at $1.55 a share, the broker will acquire all 500,000 shares and wait until there is another wave of available 500,000 share at $1.55, this limit order will not expire until all 1 million shares are acquired.
Under all or none instructions, if there are only 500,000 shares available at $1.55 a share, the broker will not buy it for the investor and instead the investor's order will expire by the end of the day. The investor will have to resubmit the next day.