Fourth market refers to the trading of securities outside of traditional exchanges, facilitated by electronic communication networks (ECNs) or other platforms. There are several synonyms that can be used to describe this type of market, including non-exchange traded market, over-the-counter (OTC) market, and dark pool. The non-exchange traded market refers to securities trading outside of organized exchanges, while OTC markets are where securities are traded directly between two parties without intermediaries. Dark pools are OTC markets where large institutional investors can trade large blocks of securities without affecting the market price. Other synonyms for the fourth market include electronic trading platforms, alternative trading systems, and direct access trading platforms.