Wikipedia
Third market
Third market in finance, refers to the trading of exchange-listed securities in the over-the-counter (OTC) market. These trades allow institutional investors to trade blocks of securities directly, rather than through an exchange, providing liquidity and anonymity to buyers
Third market trading was pioneered in the 1960s by firms such as Jefferies & Company although today there are a number of brokerage firms focused on third market trading, and more recently dark pools of liquidity.