In Dragon's Den, the investors who hear entrepreneurs' pitches are called Dragons.
The title refers to the traditional image of a dragon guarding valuable treasure. In the programme, that treasure is represented by the investors' money and business expertise. Entrepreneurs enter the Den, present their companies and answer questions before the Dragons decide whether to make an offer.
A Dragon is not simply a judge. Each investor is an independent businessperson who may offer capital, contacts, experience or a combination of these. A pitch can therefore lead to one Dragon making an offer, several Dragons competing to invest, or no investment at all.
The term is specific to this British programme and its franchise branding. Similar shows use other labels: investors on Shark Tank are called Sharks, while business competitions may use judges, mentors or panellists. The word Dragons is consequently the canonical name for the programme's investors.