Yidam is a type of deity associated with tantric or Vajrayana Buddhism said to be manifestations of Buddhahood or enlightened mind. To me, a Yidam can be anything you choose to focus your mind’s attention on during your Sadhanā (personal meditation practice) where a yogi identifies their own form, attributes and mind with those of a yidam for the purpose of transformation. This transformation is about transcending from the mundane way of life meaning leaving the daily worries and anxieties behind and a thing of past and being attuned to a higher dimensional energy that identifies with the divine sense of recognizing one’s own awakened nature.
Aham Brahmasmi