We are accustomed to thinking of Hebrew — Ivris (or Ivrit) — as the Jewish language, the holy tongue of the Bible. Apparently, the Talmud disagrees.
Generally, Jewish ritual readings must be done in Hebrew. However, reading the megillah on Purim is an exception and may be done in any language, although the sages differ whether this permission is granted to people who also understand the text’s original language. However, a Talmudic example is surprising in its list of foreign languages (Megillah 18a, Soncino tr.):
If one reads it in Coptic to the Coptics, in Hebrew to the Hebrews, in Elamean to the Elameans, in Greek to the Greeks, he has performed his obligation.
What is this Hebrew that is considered a translation of the megillah rather than the primary Biblical text? Rashi (ad loc., sv. ivris) explains that this term comes from the language of “across the river.” This is not particularly helpful in identifying the language, especially since Midrash applies that description to Avraham (Bereishis Rabbah 42:13). Apparently, later readers equated this with Aramaic. Soncino, in its footnotes, suggests that Ivri is “a kind of Aramaic spoken by the Bene Eber.” Similarly, Jastrow (sv. ivri) translates it as “a trans-Euphratean (Aramaic)” language. These are presumably based on Rashi’s comment. I am not aware of any explanation other than some sort of trans-Euphrates language and people. Artscroll translates “Hebrew to the Hebrews” as “Ivris to the Ivrites,” indicating that this is not the Jewish language for the Jewish nation but a different language for a different people.
Tosefos Ha-Rosh (ad loc., sv. ivris la-ivrim) express puzzlement over the usage here of the term Hebrew to refer to a different language. In the current context, Ashuris refers to Hebrew and Ivris to a different language. However elsewhere the Talmud equates the two.
A different Talmudic passage (Sanhedrin 21b-22a) is relevant:
Originally the Torah was given to Israel in kesav Ivri (paleo-Hebrew characters) and in the holy lanugage. It was given again to them in Ezra’s time in kesav Ashuris (Assyrian characters) and in Aramaic. Israel selected for themselves kesav Ashuris and the Hebrew language… It was taught: Rebbe said: Torah was originally given to Israel in kesav Ashuris. When they sinned it was changed to Roetz (kesav Ivri). When they repented, kesav Ashuris was reintroduced… R. Shimon ben Elazar said in the name of R. Eliezer ben Parta, who said in the name of R. Elazar Hamodai: This writing was never changed…
I discuss the historical implications elsewhere (link). However, it appears that Ivris in this context means the Hebrew language, in contrast to Aramaic.
Tosefos Ha-Rosh quote the Ri who does not cite the above passage but cites the Tosefta (Megillah 2:2), in which R. Meir was unable to find a megillah written in Ivris, which clearly means Hebrew. Ri also cites the Talmud Yerushalmi (Megillah 1:9) that equates Ivris with the holy language.
Ritva (Megillah 8b sv. ein bein) points out that the Talmud and Midrash, on various occasions, calls the Biblical language Ashuris, Ivri and Lashon Ha-Kodesh. However, Ritva continues, Ivri does not primarily refer to Heberew. He cites our passage (“Hebrew to the Hebrews”) as proof. The Rashba (Megillah 8b sv. u-tefillin) adds “וכן נהגו הכל לקרא ללשון הקדש עברי, everyone calls the holy language Ivri.” However, neither Tosefos Ha-Rosh, Ritva nor Rashba explain what else Ivri can mean. All we know is that in Talmudic times the term Ivris referred primarily to a different language and by the thirteenth century, in Rashba’s Spain, the modern usage of equating it with Hebrew was already in practice.