Atbash (a method of interchanging the first letter of the alphabet with the last, the second with the last but one and so on)
A method of reading words in which each letter in the word represents another letter, reversed in the alphabet: Aleph with Tav, Bet with Shin, Gimel with Resh, Dalet with Kuf, and so on.
("Sheshach in gematria is Babel by Atbash, Gog and Magog in gematria is seventy, which are the seventy nations"—Midrash Tanchuma (Warsaw), Parashat Korach, section 12. Likewise: "Lev Kami-Chaldeans in gematria of Atbash"—Rashi on Jeremiah 51:1. And: "At that hour, everything is turned to all sides and ponders and proceeds:... according to the written and according to the read, with the addition of the vav of usage and the bet of usage... and in the manner of Atbash... and also combined with ten sefirot"—H. Hezaz / Dorot HaRishonim, 46)
Translation
Expression (Hebrew)
אַתְּ בָּשׁ (אַ"תְּ בָּ"שׁ)
Definition (Hebrew)
שִׁיטַּת קְרִיאָה שֶׁל מִלִּים כָּךְ שֶׁכָּל אוֹת שֶׁבַּמִּלָּה מְסַמֶּלֶת אוֹת אַחֶרֶת הַהֲפוּכָה לָהּ בָּאַלְפָא-בֵּיתָא: א׳-ת׳, ב׳-ש׳, ג׳-ר׳, ד׳-ק׳, וְכוּ׳.
("שֵׁשַׁךְ בְּגִימַטְרִיָּא בָּבֶל בְּגִימַטְרִיָּא אַ"תְּ בָּ"שׁ, גּוֹג וּמָגוֹג בְּגִימַטְרִיָּא שִׁבְעִים שֶׁהֵם שִׁבְעִים אוּמוֹת"-מִדְרָשׁ תַּנְחוּמָא, פָּרָשַׁת קֹרַח סִימָן י״ב, וְכֵן: "לֵב קָמַי-כַּשְׂדִּים בְּגִימַטְרִיָּה דאַ"תְּ בַּ"שׁ" - רַשִ"י יִרְמְיָהוּ נ״א א, וְכֵן: "אוֹתָהּ שָׁעָה מִתְהַפֵּךְ לַכֹּל הַצְּדָדִים וּמְחַשֵּׁב וְהוֹלֵךְ:...לְפִי הַכְּתִיב וּלְפִי הַקְּרִי, בְּהוֹסָפַת ו׳ הַשִּׁמּוּשׁ וּבְהוֹסָפַת ב׳ הַשִּׁמּוּשׁ...וְעַל דֶּרֶךְ אַ"תְּ בָּ"שׁ...וְגַם בְּצֵירוּף עֲשָׂר סְפִירוֹת"-ח.הָזָז/ דּוֹרוֹת הָרִאשׁוֹנִים, 46)