An augur was a priest and official in the classical Roman world. His main role was the practice of augury, the interpretation of the will of the gods by studying events he observed within a predetermined sacred space (templum). The templum corresponded to the heavenly space above. The augur's decisions were based on what he personally saw or heard from within the templum; they included thunder, lightning and any accidental signs such as falling objects, but in particular, birdsigns; whether the birds he saw flew in groups or alone, what noises they made as they flew, the direction of flight, what kind of birds they were, how many there were, or how they fed. This practice was known as "taking the auspices". As circumstance did not always favour the convenient appearance of wild birds or weather phenomena, domesticated chickens kept for the purpose were sometimes released into the templum, where their behaviour, particularly how they fed, could be studied by the augur.

The augural ceremony and function of the augur was central to any major undertaking in Roman society – public or private – including matters of war, commerce, and religion. Augurs sought the divine will regarding any proposed course of action which might affect Rome's pax, fortuna, and salus (peace, good fortune, and well-being).

Although ancient authors believed that the term "augur" contained the words avis and gerō – Latin for "directing the birds", historical-linguistic evidence points instead to the root augeō: "to increase, to prosper".

Political, military and civil actions were sanctioned by augury and by haruspices.

Historically, augury was performed by priests of the college of augurs on behalf of senior magistrates. The practice itself likely comes from the neighboring region of Etruria, where augurs were highly respected as officials. Magistrates were empowered to conduct augury as required for the performance of their official duties. Magistracies included senior military and civil ranks, which were therefore religious offices in their own right, and magistrates were directly responsible for the pax, fortuna, and salus of Rome and everything that was Roman.

The presiding magistrate at an augural rite held the "right of augury" (ius augurii). The right of nuntiatio – announcing the appearance of auspicia oblativa (unexpected sign) – was reserved for the officiating augur, which would require the interruption of the proceedings then underway.

The Roman historian Livy stressed the importance of the augurs: "Who does not know that this city was founded only after taking the auspices, that everything in war and in peace, at home and abroad, was done only after taking the auspices?"

In the Regal period, which ended 509 BC, tradition holds that there were three augurs at a time; they numbered nine by the third century BC; Sulla increased their number to fifteen. By the Principate, their numbers swelled even further to an estimated 25 members.