A flamen (plural, flamines) was a specific type of priest ("sacerdos") in the ancient Roman religion and one of the oldest classes of the Roman priesthood, with origins likely predating the Republican era. These flamines, of which there were fifteen, were high-ranking members of the College of Pontiffs who administered and oversaw the various cults of the state-sponsored religion, both collectively and individually. The most important of these were the three flamines maiores ("major priests"), who each served one of the gods of the Archaic Triad: Jupiter, Mars, and Quirinus. The remaining twelve flamines minores ("lesser priests") served various minor deities, of whom little is definitively known, with two of their identities even being forgotten. While these original flamines lost most of their cultural and religious significance by the dawn of the Empire, the term flamen went on to be used in reference to priests of the cults of deified Emperors (divus).

The etymology of flamen remains obscure, and perhaps undecidable. The term is traditionally connected with the Proto-Germanic verb *blōtaną ("to sacrifice"; cf. Gothic blotan), by positing a Proto-Indo-European stem *bʰleh₂d-m(e)n- (or *bʰleh₂g-m(e)n-), which could have originally meant "sacrifice". However, the link remains uncertain since it is impossible to decide whether the Latin form reflects an earlier flă-men, flăd-men or flăg-smen.

Indo-European scholar G. Dumézil attempted to link the term to the Sanskrit word brahman. Dumézil himself notes that the etymology has problems in terms of phonological shifts, and the cognates have not been universally accepted by modern scholars. Andrew Sihler considers the claim that flamen might be a cognate of the Vedic term to be as plausible. He notes that the hypothesis of a connection to Gothic blotan and via Proto-Indo-European *bʰleh₂d-m(e)n- is equally plausible.

By the time of the religious reformations initiated by Emperor Augustus, the origins and functions of a number of gods resident in Rome were considered confusing and archaic, even to the Romans themselves. The age and relative obscurity of some of the deities assigned a flamen (such as Falacer, Palatua, and Volturnus) suggests that this class of the priesthood dates back to the early days of Rome, at least as far back as the Roman Kingdom. The Romans themselves even credited their foundation to Numa Pompilius, the legendary second King of Rome. According to the historian Livy, in order to fulfill the more intensive religious duties originally performed by the King, Numa created the offices of the three flamines maiores and assigned each a fine robe of office and a curule seat. The flamines were circumscribed by a series of highly restrictive taboos, as in the case of the flamen Dialis, the stability of Rome itself was believed to depend on his continued purity and holiness. In fact, the most detailed surviving records of the flamines often focus on the practices and traditions associated with the flamen Dialis, the highest-ranking and most significant of the flamines.

Some modern scholars have raised the possibility that the roles of the Roman flamines may have extended beyond simply overseeing the cults of their respective deities, even hypothesizing that they may have represented and provided religious services to certain segments of the Roman population. This theory is supported by the existence of a flamen among the Arval Brethren (the flamen Arvalium), as well as the flamines of the thirty curiae that divided early Rome (flamines curiales). Based on descriptions from some Roman sources, the three flamines maiores may have collectively represented the entire Roman populace, as evidenced by their common sacrifices to the goddess of faithfulness Fides. By the Republican era, the maiores were collectively invoked both in treaties composed by the fetiales and in the devotio oath taken by generals during times of unrest.

By the Imperial era, the term flamen seems to have mostly been used in reference to the priests of the Imperial cult, whose regulations and standards appear to have been modeled after those of the flamen Dialis. This new incarnation of the priesthood aided in the spread and homogenization of the Imperial cult throughout the growing Roman Empire, with priests being assigned to serve certain cities and provinces, and in some regions specifically dedicated to the cult of Augustus (the divi filius, or more specifically the divi Iuli filius). While these priests were still given the antiquated title of flamen in most provinces and regions, others used the more general and widely-recognized sacerdos (plural, sacerdotes), as was the case with the Sacerdotes Augustales.

In post-Antiquity, the word "flamen" (plural, "flamens") became a general term for any priest who serves a specific deity.

Since the early days of the flaminates, the Roman populace took an active role in the selection of a new flamen, nominating fellow citizens believed to be worthy of the position. Originally these nominations were handled by the Curiate Assembly, the oldest legislative gathering in the city, ultimately selecting and consecrating each new flamen. However, following the passing of the Roman law "Lex Domitia de sacerdotis" ("The Domitian Law Regarding the Priesthood") in 104 BC, which formalized the selection processes for Roman priests, nominations for the flaminates were now handled by the more egalitarian Tribal Assembly. They would then be provided to the pontifex maximus, the head of the Pontifical College, who "scrutinized each candidate's qualifications in order to ensure that he...[was] fit to serve." After a new flamen (and, for maiores, also his wife) was chosen, they then had to participate in a traditional Roman ceremony known as the captio, to ensure the gods would accept this new flamen as their representative to the city. An augur would ask the appropriate deities for a blessing, and the gods would respond by providing the proper signs to those present. These ceremonies were known as comitia calata ("callate assemblies") and they were performed on the Capitoline Hill.