A security, such as a stock, is unlisted when it does not meet the listing requirements or pay the listing fee of any of the organized exchanges or markets.
Unlisted stock may be traded over-the-counter (OTC), however, and its price and volume may be tracked in the Pink Sheets or on the OTC Bulletin Board (OTCBB).
In most cases, unlisted stocks are thinly traded because they do not get much attention from the media or financial analysts, and so may be too risky for many investors.