In Dragon's Den, the investors are called Dragons.
Entrepreneurs enter the studio and present their businesses to a panel of established businesspeople. The Dragons ask about sales, costs, valuation, intellectual property and growth before deciding whether to make an offer. A successful deal normally involves investment in return for an agreed percentage of the company.
The word “Dragons” is the defining British terminology for the format. It is not merely a nickname for wealthy guests; it is the programme's official name for its investor panel and appears in the title itself.
The same underlying format is known by different names in different countries. The United States version uses “sharks,” which is why international viewers sometimes substitute that word when discussing the British programme. In the UK series, however, the canonical term is Dragons.