Priests of the Vedic religion are officiants of the yajna service. As persons trained for the ritual and proficient in its practice, they were called ?tvij ("regularly-sacrificing"). As members of a social class, they were generically known as vipra "sage" or kavi "seer". Specialization of roles attended the elaboration and development of the ritual corpus over time. Eventually a full complement of sixteen ?tvijas became the custom for major ceremonies. The sixteen consisted of four chief priests and their assistants.
The older references uniformly indicate the hot? as the presiding priest, with perhaps only the adhvaryu as his assistant in the earliest times. The phrase "seven hotars" is found more than once in the Rigveda. Hymn 2.1.2 of Rigveda states it as follows,
? ? ? ? ? ? 
Thine is the Herald's task and Cleanser's duly timed; Leader art thou, and Kindler for the pious man. Thou art Director, thou the ministering Priest: thou art the Brahman, Lord and Master in our home.-- Rigveda 2.1.2
The above hymn enumerate the priests as the hot?, pot?, ne, agn?dh, prash?st? (meaning the maitr?varuna) and adhvaryu.
The rgvedic Brahmanas, Aitareya and Kausitaki, specify seven hotrakas to recite shastras (litanies): hot?, br?hman?cchamsin, maitr?varuna, pot?, ne, agn?dh and acch?v?ka. They also carry a legend to explain the origin of the offices of the subrahmanya and the gr?vastut.
The requirements of the fully developed ritual were rigorous enough that only professional priests could perform them adequately. Thus, whereas in the earliest times, the true sacrificer, or intended beneficiary of the rite, might have been a direct participant, in Vedic times he was only a sponsor, the yajam?na, with the hot? or brahman taking his stead in the ritual. In this seconding lay the origins of the growing importance of the purohita (literally, "one who is placed in front"). It was not unusual for a purohita to be the hot? or brahman at a sacrifice for his master, besides conducting other more domestic (g?hya) rituals for him also. In latter days, with the disappearance of Vedic ritual practice, purohita has become a generic term for "priest".
In the systematic expositions of the shrauta sutras, which date to the fifth or sixth century BCE, the assistants are classified into four groups associated with each of the four chief priests, although the classifications are artificial and in some cases incorrect:
This last classification is incorrect, as the formal assistants of the brahman were actually assistants of the hot? and the adhvaryu.[clarification needed]
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Comparison with the sacred texts of Zoroastrianism, a distinct religion with the same origins, shows the antiquity of terms for priests such as *atharwan (Vedic atharvan; cognate to Avestan rauuan / a?aurun) and *zhautar (Ved. hotar; Av. zaotar) "invoker, sacrificer". While *zhautar is well understood, the original meaning of *atharwan is unknown. The word atharvan appears in the Rig Veda (e.g., in RV 6.16.13 where Agni is said to have been churned by Atharvan from the mind of every poet). In the Younger Avesta, rauuan / a?aurun appears in a context that suggests "missionary," perhaps by metathesis from Indo-Iranian *arthavan "possessing purpose." However, a recent theory indicates that Proto Indo-Iranian *atharwan likely represents a substrate word from the unknown language of the BMAC civilization of Central Asia. It can be analyzed as BMAC *athar- plus the Indo-Iranian possessive suffix *-wan, in which case *atharwan would be "one who possesses *athar". Though the meaning of *athar is unknown, Pinault speculates that it meant "superior force" and connects it to the Tocharian word for "hero". In the Upanishads, atharvan appears for example in atharv?ngiras, a compound of atharvan and angiras, either two eponymous rishis or their family names.
In present-day Indian Zoroastrian (Parsi) tradition the word athornan is used to distinguish the priesthood from the laity (the behdin). These subdivisions (in the historical Indian context, castes), and the terms used to describe them, are relatively recent developments specific to Indian Zoroastrians and although the words themselves are old, the meaning that they came to have for the Parsis are influenced by their centuries-long coexistence with Hinduism. It appears then that the Indian Zoroastrian priests re-adopted the older rauuan / a?aurun (in preference to the traditional, and very well attested derivative ?sron) for its similarity to Hinduism's atharvan, which the Parsi priests then additionally assumed was derived from Avestan ?tar "fire". This folk-etymology, which may "have been prompted by what is probably a mistaken assumption of the importance of fire in the ancient Indo-Iranian religion" (Boyce, 1982:16).
The division of priestly functions among the Hotar, the Udgatar and the Adhvaryu has been compared to the Celtic priesthood as reported by Strabo, with the Druids as high priests, the Bards doing the chanting and the Vates performing the actual sacrifice.