The Geonim (Hebrew: גאונים, plural of gaon; sing. “Excellency”) were the heads of the two principal Babylonian Jewish academies, at Sura and Pumbedita, during the period roughly 589-1038 CE.1) They were the formal title-holders of the yeshivot founded in the era of the Amoraim and continued through the Savoraim, and during the geonic period they were widely recognized as the highest legal and religious authorities for Jewish communities across the diaspora, in distinction from the Exilarch (Resh Galuta), who held secular and political authority over Jews under Islamic rule.2) The geonic period is conventionally dated from the end of the sixth century, when the title gaon came into use, to the death of Hai Gaon around 1038, after which the institution declined.3)
The exact scope of “geonic” authority is itself a matter of some historical disagreement – later sources extended the title honorifically to academy heads in Baghdad, Damascus, Egypt, and the Land of Israel, and eventually to any sufficiently learned rabbi – but the core historical usage refers specifically to the heads of Sura and Pumbedita. See geonim-title-scope-debate.
The Geonim's central historical role was the interpretation, transmission, and application of the Babylonian Talmud, which had been edited and closed by the Savoraim shortly before the geonic period began.4) Whereas the Amoraim had produced the Talmud through their disputation and the Savoraim had fixed its text, the Geonim's task became explaining and applying that fixed text to new circumstances.5) Jewish communities throughout the Islamic world and beyond sent legal and religious questions to the academies; the resulting body of written answers is known as responsa literature, and constitutes the primary surviving source for geonic legal thought.6)
Geonic authority was not static. In the early centuries of the period, most inquiries came from Babylonia and neighboring regions, but from roughly the mid-tenth century onward, distant communities increasingly developed their own local scholars and stopped routing questions to Babylonia, gradually eroding the academies' practical primacy even as their prestige persisted.7) Internally, Sura held earlier primacy, but Pumbedita's standing grew over the latter part of the period.8) For the institutional and political history in more detail, see geonim-history.
The geonic period also produced sustained engagement with the Karaite movement, which rejected the binding authority of the Oral Torah and rabbinic tradition in favor of direct scriptural interpretation.9) Saadia Gaon of Sura (892-942) is the figure most associated with systematic anti-Karaite polemic, though the question of how earlier Geonim responded to Karaism – whether through silence, indirect measures, or direct confrontation – is itself debated among historians. See geonim-karaite-debate.