The term mixed lot refers to an order that contains a combination of both round and odd lot orders. A mixed lot for stock is any order that is greater than 100 shares, but not a multiple of 100 shares.
The standard trading unit for common stocks is 100 shares, while it's $100,000 for bonds. Trading in a multiple of a standard unit is referred to as a round lot, while trading fewer shares than a round lot is referred to as an odd lot. A mixed lot contains a combination of both odd and round lots. For example, an order for 1 to 99 shares of stock is an odd lot order. Trading 100 or 200 shares of stock is a round lot order. If the investor trades 125 shares of stock, the trade would be considered a mixed lot order.
Broker commissions for trades are typically stated in terms of a standard trading unit for a security. In the case of common stock, the fee charged by a broker to execute a trade would be stated as a minimum fixed amount for a standard trading unit. In the past, traders would try to avoid placing orders for odd or mixed lots, since the commission can have a significant impact on the trader's return on investment.
Mixed lots are treated differently by exchanges too, since the investor is not trading in a standard unit. For example, these orders will not affect the displayed bid / ask prices, and they are not posted to tickers. Finally, since the trade is not in a standard unit, the execution of the transaction is also delayed.