What is the meaning of Third market?

Exchange-listed securities, such as those that are traded on the New York Stock Exchange (NYSE) or the American Stock Exchange (AMEX), are also bought and sold off the exchange, or over the counter (OTC), in what is known as the' Third market'. Typically, third-market transactions are large block trades involving securities firms (that may or may not be members of the exchange) and institutional investors, such as investment companies and pension funds. With the advent of electronic communications networks (ECNs), however, more investors are buying and selling in this way. Among the appeals of the third market are speed, reduced trading costs, and anonymity.