A listed security is a financial instrument, such as a bond or stock, that is traded on an exchange. Listing is quite valuable to investors, since they can more easily buy and sell securities at a market price. It is time-consuming and expensive for a company to gain a listing, since it must have its books audited, comply with numerous requirements of the selected exchange, and apply to have its securities registered with the Securities and Exchange Commission (SEC). The SEC's requirements are extensive, so the cost of filing can put a securities listing out of the reach of smaller and less well-funded organizations.
A security that is not listed can be traded in the over-the-counter market, which is much less liquid.