yidam

In Vajrayana Buddhism, a Yidam (Tibetan) or Ishta-devata (Sanskrit) is a fully enlightened being who is the focus of personal meditation, during a retreat or for life. The term is often translated into English as meditational deity. A Yidam is an enlightened being with whom one identifies during meditation: one perceives his or her own Buddha nature through such identification. Some common Yidams include Hayagriva, Vajrakilaya (Dorje Phurba), Samputa, Guhyasamaja, Yamantaka, Hevajra, Kurukulle, Cakrasamvara, Vajrayogini, and Kalachakra. Also, other enlightened beings such as the regular forms of the Buddhas, Bodhisattvas, Guru Rinpoche, certain Dharmapalas, Wealth Deities, and yab-yum representations, among others, can also be practiced as a Yidam.
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A bodhisattva or other "deity" assigned to a Vajrayana monk by his guru as his personal guide and protector. Once established, this link will last the monk's lifetime, and will help him work towards attaining enlightenment.