To define the concept of the yidam is to approach the essence of Tibetan Buddhism. The yidam is a special deity one works with in meditation as a means towards recognizing one's own awakened nature. The word is said to be a contraction of yid kyi dam tshig, which essentially means to bind one’s mind (yid) by oath to a deity who embodies enlightened mind.
In Tibetan Buddhism there are innumerable kinds of deities, but the yidam is defined by the very distinctive role it plays in meditation. Yidams may be sambhogkaya buddhas, tantric deities, bodhisattvas, dharma protectors or historical figures. In all cases, the yidam is the very manifestation of enlightenment, and every aspect of it is ultimately meaningful. The yidam is one of the so-called Three Roots that are the objects of refuge in vajrayana: the guru, the yidams, and the protectors and dakinis. As such, it is said to be the root of spiritual power or accomplishment (Skt. siddhi). How does that work?