Ktav Ashuri

Ktav Ashuri (, ' "Assyrian script"; also Ashurit) is the traditional Hebrew language name of the Hebrew alphabet, used to write both Hebrew and Jewish Babylonian Aramaic.
In halakha, tefillin (phylacteries) and mezuzot (door-post scripts) can only be written in Ashurit.
History.
"Ktav Ashuri" is the term used in the Talmud; the modern Hebrew term for the Hebrew alphabet is simply "Hebrew alphabet". Consequently, the term "Ktav Ashuri" refers primarily to a traditional calligraphic form of the alphabet used in writing the Torah.
The name reflects the fact that the Hebrew alphabet is was derived from the Aramaic alphabet (), it thus refers to "the Aramaic alphabet as used in Judaism", and is sometimes referred to as the "Assyrian script."
The name contrasts with the name "Libonaa" (or "Liboni") given to the Samaritan alphabet, and by extension the Paleo-Hebrew alphabet. This name is most likely derived from "Lubban", i.e. the script is called "Libanian" (of Lebanon), although it has also been suggested that the name is a corrupted form of "Neapolitan", i.e. of Nablus.
According to the Talmud, Ezra was the first to mandate that the sefer Torah be written in the Assyrian alphabet rather than in the Paleo-Hebrew alphabet used formerly and permitted that the Book of Daniel be composed in Aramaic. The term "Ashurit" is often used in the Babylonian Talmud to refer to the modern-Hebrew writing script.
According to the Babylonian Talmud, the Torah was given by Moses in the Assyrian alphabet, later changed to the Paleo-Hebrew script, and, again, the Ashurit script during the time of Ezra. The matter, however, remains disputed, some Sages holding the view that the Torah was originally inscribed in the Old Hebrew (Paleo-Hebrew) script.
Mention of the Ashuri script first appears in rabbinic writings of the Mishnaic and Talmudic periods, referring to the formal script used in certain Jewish ceremonial items, such as sifrei Torah, tefillin, mezuzot and the Five Megillot. Also sometimes called the "square" script, the term is used to distinguish the Ashuri script from the Paleo-Hebrew alphabet.
The Talmud gives two opinions for why the script is called "Ashuri": either because the Jews brought it back with them when they returned from exile in Assyria; or alternatively, this script was given at Mount Sinai and then forgotten and eventually revived, and received its name because it is "meusheret" (beautiful/praiseworthy or authorized).