Odd lots are referring to anything under the normal trading unit which is generally set at 100 shares for stocks. This can include orders for 1-99 shares of a particular stock and is in contrast to more commonly used round lots which are divisible evenly by 100. Generally, odd lot orders can cost more and take longer to complete as they are considered less attractive to larger trading players.

One reason why odd lots may occur is when companies announce a reverse stock split or initiate a dividend reinvestment plan. A reverse stock split requires shareholders to exchange multiple shares in the company for fewer, but larger stake in the company, which can result in an odd lot order. Meanwhile, many companies offer dividend reinvestment plans which enable shareholders to reinvest the cash dividends they receive into additional shares of the company. In some cases, this could lead to holdings of an odd lot.

For large companies, odd lots can generally be considered immaterial and thus can be wiped out by a buyout in which the company could offer the shareholder a premium or additional stock to bring the holdings back up to a round lot.

In summary, odd lots refer to orders for stocks that are less than the normal unit of trading and can result from a reverse stock split or dividend reinvestment plan. Such orders can cost more and take longer to complete, and for large companies can be seen as insignificant and eliminated by providing the shareholder a premium or bigger stake in the company.