Deity yoga is the central meditative practice of tantric Buddhism, the Vajrayana of Tibet, Mongolia and, in a related form, Japanese Shingon. In it the practitioner visualises a buddha or bodhisattva in precise detail, complete with colour, ornaments, implements and surrounding mandala, and then, crucially, dissolves the ordinary sense of self and arises in imagination as that deity, identifying with its enlightened body, speech and mind. The logic is that of the tantric path: rather than approaching enlightenment as a distant goal to be built up over ages, the practitioner "takes the result as the path," rehearsing the awakened state directly by assuming the form of a being who already embodies it.
The practice is demanding and is never, in the tradition, undertaken casually. It requires an empowerment, a formal initiation conferred by a qualified master that authorises the practitioner to visualise a given deity, together with the transmission of the associated mantra and instructions. Without that authorisation the practice is held to be not merely ineffective but improper.