Yidam
The Yidam, or Ishta-devata, is a personal meditation deity. The Sanskrit word iṣṭadevatā or iṣṭadevaḥ is defined by V. S. Apte as "a favorite god, one's tutelary deity." Though this term is used in many popular books on Buddhist Tantra, the term işţadevatā has not been attested in any Buddhist tantric text in Sanskrit. The unrelated Tibetan version of the term, possibly of entirely native origin, is yi-dam is said to be a contraction of Tib. yid-kyi-dam-tshig, meaning "samaya of mind"- in other words, the state of being indestructibly bonded with the inherently pure and liberated nature of mind.
The Ishta-devata of Hinduism is an aspect of God for personal worship. In Buddhism, a Yidam is a manifestation of enlightenment and make take the form of Sambhogakāya Buddhas, tantric deities, bodhisattvas, Dharma protectors or other historical figures.
Read more about this topic: Buddhist Mythology