A type of order for a number of securities that is not a round (or whole) lot order amount. This type of order is comprised of a round lot order and an odd lot order. A round lot is the exchange-established trading unit, which defines the interval at which securities typically should be traded. An odd lot is an order that falls below the initial round lot amount.
Stocks typically trade in round lots of 100, which means orders made in these intervals are traded easily between parties. An odd lot would be all orders for 99 shares or fewer.

If an investor wanted to buy 425 shares he or she would use a mixed lot order, which is broken into an round lot order for 400 shares (4 x 100 round lots) and an odd lot order for 25 shares.