A round lot is the standard, generally accepted number of units in which a security is sold. For example, a round lot for the trading of shares is some multiple of 100 shares. In bond trading, a round lot is usually $100,000 of bonds. Traders encourage the use of round lots, since they expend less effort to process the orders and so may charge less to process these order sizes.
Any other amount of a trade than a round lot is classified as an odd lot.