Hinduism: Within Hinduism, an Ishta-Deva or Ishta Devata (Sanskrit: iṣṭa-deva(tā), literally "cherished divinity" from iṣṭa "desired, liked, cherished, preferred" and devatā "godhead, divinity, tutelary deity" or deva "deity") is a term denoting a worshipper's favourite deity.
It is especially significant to both the Smarta and Bhakti schools wherein practitioners choose to worship the form of God which inspires them the most. Within Smartism, one of five chief deities are selected. Even in denominations that focus on a singular concept of God, such as Vaishnavism, the Ishta Deva concept exists. For example, in Vaishnavism, special focus is given to a particular form of Lord Vishnu or one of his Avatars (i.e. Krishna or Rama), and similarly within Shaktism, focus is given to a particular form of the Goddess such as Parvati or Lakshmi. The Swaminarayan sect of Vaishnavism has a similar concept, but notably differ from practically all Vaishnavite schools in holding that Vishnu and Shiva are different aspects of the same God.
Buddhism: Yidam is a type of deity associated with tantric or Vajrayana Buddhism said to be manifestations of Buddhahood or enlightened mind. During personal meditation (sādhana) practice, the yogi identifies their own form, attributes and mind with those of a yidam for the purpose of transformation. Yidam is sometimes translated by the terms "meditational deity" or "tutelary deity". Examples of yidams include the meditation deities Chakrasamvara, Kalachakra, Hevajra, Yamantaka, and Vajrayogini, all of whom have a distinctive iconography, mandala, mantra, rites of invocation and practice.
In Vajrayana, the yidam is one of the three roots of the "inner" refuge formula and is also the key element of Deity yoga since the 'deity' in the yoga is the yidam.