Acharonim (Hebrew: [(?)a?(a)?o'nim]; Hebrew: ? A?aronim; sing. , A?aron; lit. "last ones") in Jewish law and history, are the leading rabbis and poskim (Jewish legal decisors) living from roughly the 16th century to the present, and more specifically since the writing of the Shulchan Aruch (Hebrew: , "Set Table", a code of Jewish law) in 1563 CE.
The Acharonim follow the Rishonim, the "first ones"--the rabbinic scholars between the 11th and the 16th century following the Geonim and preceding the Shulchan Aruch. The publication of the Shulchan Aruch thus marks the transition from the era of Rishonim to that of Acharonim.
Consequences for Halakhic change
The distinction between the Acharonim, Rishonim and Geonim is meaningful historically. According to the widely held view in Orthodox Judaism, the Acharonim generally cannot dispute the rulings of rabbis of previous eras unless they find support from other rabbis in previous eras. Yet the opposite view exists as well:
In The Principles of Jewish Law Orthodox Rabbi Menachem Elon wrote:
The Principles of Jewish Law
-- [such a view] "inherently violates the precept of Hilkheta Ke-Vatra'ei, that is, the law is according to the later scholars. This rule dates from the Geonic period. It laid down that until the time of Rabbis Abbaye and Rava (4th century) the Halakha was to be decided according to the views of the earlier scholars, but from that time onward, the halakhic opinions of post-talmudic scholars would prevail over the contrary opinions of a previous generation. See Piskei Ha'Rosh, Bava Metzia 3:10, 4:21, Shabbat 23:1
Hilkheta Ke-Vatra'ei can be interpreted in a way such that the Orthodox view does not constitute a contradiction, with an appeal to understand it within the greater context of Torah. While authority may go to the scholars of a later generation within a particular era, the Talmud does not allow scholars of a later era to argue with scholars of an earlier era without support from other scholars of an earlier era.
This is displayed in "hundreds of instances" in the Talmud in which Amora'im are challenged by Tanna'itic sources with the term and the Amorai'm unable to "deflect the challenge". An Amora called Rav is challenged by Tannai'tic sources "and is vindicated by the statement, Rav tanna hu upalig"- "Rav is a Tanna and disagrees (in Eiruvin 50b, Kesubos 8a, and elsewhere). Similar case for Rav China, a borderline Tanna in Bava Metzia 5a. This clearly implies that the only reason they are able to get away with disagreeing is because they are Tannaim. There are "only a handful of possible exceptions that the Amora'im did not, in fact argue with the Tanna'im."
The question of which prior rulings can and cannot be disputed has led to efforts to define which rulings are within the Acharonim era with precision. According to many rabbis the Shulkhan Arukh is from an Acharon. Some hold that Rabbi Yosef Karo's Beit Yosef has the halakhic status of a work of a Rishon, while his later Shulkhan Arukh has the status of a work of an Acharon.