Every society runs on a large body of behaviour that is not law and not religion, and that nobody polices: how a guest is received, what is owed to a cousin, what a host may ask of a visitor and a visitor of a host. Aryaman is the Vedic name for that. He is the third of the great Adityas after Varuna and Mitra, invoked constantly in their company and almost never given a hymn of his own, and his sphere is the customary obligation rather than the sworn contract or the cosmic law.
His name is related to arya, the term a Vedic community applied to itself, and it carries the sense of the companion or the one belonging to the group. That is why the obligations he covers are internal ones. Varuna deals with the offence, Mitra with the agreement between parties, and Aryaman with the ordinary conduct of people who are already on the same side.
He is also, on the evidence of the hymns, generous. Formulas ask him for prosperity, for a good road, for freedom from narrowness, and he appears in the company of Bhaga, the apportioner, in requests for a fair share. Comparative work has proposed Iranian and Celtic relatives for the name, and the proposals are argued rather than settled, so the atlas keeps them as comparison and not as fact. What is not in doubt is his function inside the Vedic material: he is where the tradition put the things a community does because that is how they are done.