Unlisted Security

A security that is not traded on an exchange, usually because of an inability to meet listing requirements.

Investopedia Says:
An unlisted security is also known as an "over-the-counter" (OTC) security. In the UK the term "unquoted" is used.

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Security that is not listed on an organized exchange, such as the New York Stock Exchange, the American Stock Exchange or the Regional Stock Exchanges and is traded in the Over the Counter market.

A stock or bond not listed on a stock exchange and therefore traded only in the over-the-counter market.

The noun has one meaning:

Meaning #1: a security traded in the over-the-counter market
  Synonyms: over the counter security, OTC security
  Antonym: listed security (meaning #1)