In the Avesta, airyaman (or airiiaman) is both an Avestan language common noun and the proper name of a Zoroastrian divinity.
The common noun is a theological and social term literally meaning "member of (the) community or tribe." In a secondary development, the common noun became the proper name of a divinity Airyaman, who is the yazata of health and healing.[*]
In Zoroastrian tradition, Avestan Airyaman is Middle Persian Erman (Ērmān).
The divinity Airyaman does not appear in the Gathas, the oldest texts of Zoroastrianism and considered to have been composed by Zoroaster himself. In the few instances where the term does appear (Yasna 32.1, 33.3, 33.4, 49.7), airyaman is a common noun denoting the social division of priests.
According to a cosmogonical story preserved in the Vendidad, not long after Ahura Mazda had created the world, Angra Mainyu unleashed innumerable sicknesses upon it. In response, Ahura Mazda requested Manthra Spenta, Sraosha and Airyaman to find a cures for them, promising each that he would reward them and bless them with Dahma Afriti. With Airyaman's assistance, Ahura Mazda then brought 10,000 plants to the earth, so providing Thraetaona with the means to cure the world of all ills (Vendidad 22.5).
Airyaman is closely associated with Asha Vahishta, the Amesha Spenta of "Best Truth" (or "Best Righteousness"). In Vendidad 20.11 and in Yasht 2 (dedicated to the seven Amesha Spentas), he is described as "following" asha, which is what Asha Vahishta is the hypostasis of. The third Yasht, which is nominally a hymn to Asha Vahishta is for the greater part a praise of the airyaman ishyo, which in Zoroastrian tradition is considered to be an invocation of the divinity Airyaman.
Like the truth/order (asha) that is preserved through the proper recitation of prayer, "Airyaman does not heal by means of herbs and drugs, medicine and surgery, but by the holy spells." Although Airyaman does not have a day-name dedication in the Zoroastrian calendar, he is invoked together with Asha Vahishta on the third day of the month (Siroza 2.