Atar (Avestan ātar) is the Zoroastrian concept of holy fire, sometimes described in abstract terms as "burning and unburning fire" or "visible and invisible fire" (Mirza, 1987:389).
In the Avestan language, ātar is an attribute of sources of heat and light, of which the nominative singular form is ātarš, source of Persian āteš (fire). It was once thought to be etymologically related to the Avestan āθrauuan (Vedic atharvan), a type of priest, but that is now considered unlikely (Boyce, 2002:16). The ultimate etymology of atar, previously unknown (Boyce, 2002:1), is now believed to be from the Indo-European *hxehxtr- 'fire'. This would make it related to the Latin ater (black) and either a loan source or cognate of the Serbian ватра / vatra (fire) (also of the Romanian vatră (hearth, home, fireplace), of possible Dacian or Paleo-Balkanic origin), which sets Serbian apart from other Slavonic languages, e.g. Russian (огонь = "fire"; Serbian also has огањ - "fire", but it is mostly poetic). The yazata Atar may not be of Indo-Iranian origin (Dhalla 1938:174).
In later Zoroastrianism, atar (in middle Persian: ādar or ādur) is iconographically conflated with fire itself, which in middle Persian is ataksh, one of the primary objects of Zoroastrian symbolism.