Ahura is the Avestan language designation for a class of divinity, adopted by Zarathustra (Zoroaster) from prehistoric proto-Indo-Iranian religion. The term was subsequently inherited by Persian mythology, where ahura () has the same meaning as in Zoroastrianism.
The meaning of asura in the Vedic religions is the moral opposite of the meaning of ahura in Zoroastrianism (and in all Iranian languages). Although Vedic religions are not as polarized on issues of good versus evil as Zoroastrianism is, the Vedic devas are more propitious than not, while the asuras are more easily associated with death and destruction. In Zoroastrianism, where the battle between good and evil is a distinguishing characteristic of the religion, the ahuras are wholly benevolent, and the daevas are wholly malevolent.
The process by which the terms came to have different meanings remains unclear, but the use of ahura to designate the 'right' divinities and daeva to designate the 'wrong' divinities was firmly established by the time the Gathas, the oldest and most sacred of the texts of the Avesta, were composed (roughly contemporary to the Brahmana period of Vedic Sanskrit).
In the oldest of the Vedic texts, asura is used to denote the "older gods" presiding over the moral and social phenomena of the primeval universe, while the devas are the "younger gods" who preside of nature and the environment (Rig Veda 10.124.3). In the Vedic account of creation, some of these "older gods" went over to the "younger gods", so joining the ranks of the devas, and the remaining asuras were exiled to the nether world. While this distinction between asuras and asuras-who-became-devas is preserved in the texts of the Rig Veda, in later texts, asura is simply an epithet to categorize all non-devas, or alternately, those who are opposed to the devas.
In the Gathas, which are thought to have been composed by Zoroaster himself, the prophet exhorts his followers to pay reverence to only the ahuras, and to rebuff the daevas and others who act "at Lie's command". In the Fravaraneh, the Zoroastrian credo summarized in Yasna 12.1, the adherent declares: "I profess myself a Mazda worshiper, a follower of the teachings of Zoroaster, rejecting the daevas, ... " This effectively defines ahura by defining what ahura is not.
In the Gathas, Zoroaster does not specify which of the divinities (aside from Ahura Mazda) he considers to be an ahura. While Ahura Mazda is "the mightiest Ahura" (Yasna 33.11) and Zoroaster speaks of "the Wise one and the (other) Ahuras" (Yasna 30.9, 31.4), specifics are not provided.
In later texts of the Avesta however, in particular in the Yashts, the texts dedicated to the individual divinities of the Zoroastrian hierarchy, Mithra is frequently named as one of the ahuras. In Achaemenid era iconography, Mithra, Anahita and the Baga (*vouruna, according to Boyce) are recorded as ahuras.