Over-the-counter (OTC), also known as off-exchange trading, refers to trading without a regulated exchange trading floor. Stocks, bonds, and other securities of companies that are not listed on a formal exchange can be traded over the counter via a broker-dealer network as opposed to on a centralized exchange.
In the U.S., the Over the Counter Bulletin Board (OTCBB) or the Pink Sheets listing services are popular inter-dealer quotation systems and networks for trading.
Forex trading also takes place in the OTC market, which allows traders to trade 24 hours a day.