Definition of 'Non-Marketable Security'
Any type of security that is difficult to buy or sell because it does not trade on a normal market or exchange. These types of securities trade over the counter (OTC) or in a private transaction. Finding a party with which to transact business is often difficult; in some cases, these securities can't be resold due to regulations surrounding the security.
Investopedia explains 'Non-Marketable Security'
Some examples of non-marketable securities are savings bonds, series (A, B, EE, etc.) bonds and private shares. The U.S. government offers both marketable and non-marketable securities to the public. Marketable securities, such as treasury bills and bonds can be purchased and resold to the public. But non-marketable securities, such as savings bonds, must be held by the holder until maturity and can't be resold to another party.
Limited partnership (LP) interests are often difficult, if not impossible to resell.