What is "yidam"

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.