The business pitch room used by the investors in Dragon's Den is called the Den.
The title of the British programme refers directly to this setting, where entrepreneurs stand before the investor panel and present their companies. The room is designed as a formal, high-pressure meeting space rather than a conventional television stage, reinforcing the idea that contestants are entering the Dragons' territory.
A typical pitch includes the entrepreneur's requested investment, the percentage of equity offered and a valuation of the business. The Dragons then ask questions and may make individual or joint offers. If no investor agrees to a deal, the contestant leaves without investment.
The phrase is sometimes used loosely for the entire programme, but “the Den” specifically names the pitch environment. It is not an investor, company or separate spin-off series.