Definition of 'Round Lot'
A group of 100 shares of a stock, or any group of shares that can be evenly divided by 100, such as 500, 2,600 or 14,300. A round lot has historically been the smallest order that can be placed through an exchange. However, "round-lot one" now allows for the execution of orders as small as one share on some exchanges. A round lot may also be referred to as a "normal trading unit".
Investopedia explains 'Round Lot'
A lot consisting of fewer than 100 shares or a lot that cannot be evenly divided by 100 is called an odd lot. Sometimes odd lots are combined, or "bunched", into round lots to facilitate trading. A mixed lot consists of both a round lot and an odd lot. An order of 198 shares would be considered a mixed lot.
In bond trades, a round lot is usually $100,000 worth of bonds.