In Buddhism, a yidam (Sanskrit, devata) is generally considered a symbolic representation of our fullest potential as a deity: it is an illustration of what our seed (buddhadhatu) can become. It can then be said that the yidam is our Innermost Being, symbolized in the form of a deity. This is the basis of Tantric Buddhist Deity Yoga. Practictioners of this method meditate on the deity in order to experience its true nature as a manifestation of ultimate reality.
This idea is comparable to the Hindu term इष्टदेवता ishtadevata and Hindu Bhakti Yoga.
"The practical aspects of philosophy of Guru-Bhakti Yoga is to realise the oneness of Guru with his Ishta Devata." —Swami Shivananda
"Vedantins try to fix the mind on Atman. This is their Dharana. Hatha Yogins and Raja Yogins concentrate their mind on the six Chakras. Bhaktas concentrate on their Ishta Devata [Yidam]. Other objects of meditation are described under Trataka and Laya Yoga. Concentration is necessary for all the aspirants." —Swami Shivananda