Shem HaMeforash

ב וְשִׁבְעָה שֵׁמוֹת הֶן--הַשֵּׁם הַנִּכְתָּב יוֹד הֵא וָאו הֵא וְהוּא הַשֵּׁם הַמְּפֹרָשׁ, אוֹ הַנִּכְתָּב אֶלֶף דַּאל נוּן יוֹד, וְאֵל, וֶאֱלוֹהַּ, וֵאלֹהִים, וְאֶהְיֶה, וְשַׁדַּי, וּצְבָאוֹת. כָּל הַמּוֹחֵק אַפִלּוּ אוֹת אַחַת מִשִּׁבְעָה שֵׁמוֹת אֵלּוּ, לוֹקֶה

Here the Rambam states pretty clearly his opinion that the shem hameforash is the tetragrammaton - the shem yud key vav key. He reiterates this in Moreh Nevuchim 1:61,62,63. There he also cites the gemara in Kiddushin 71a that states that there also exists a twelve letter name of Hashem as well as a 42 letter name. The Rambam there says that these longer names were probably more like phrases that described certain attributes of Hashem. Only the shem hameforash - the four letter yud key vuv key - is a name that can be said to denote more than mere attributes of Hashem. In some way that name even describes Hashem's essence. The Rambam says that we don't know one hundred percent what that name means, but it probably means something along the lines of "absolute existence".

There is a gemara in Sanhedrin 60a that discusses the laws of Megadef. In that gemara there is one part where the gemara says that even when one uses the four letter name of Hashem it is megadef. To which the gemara asks, what is the chiddush? The gemara answers that you may have thought you need the "shem rabba" the great name. So, the chiddush is even the four letter name suffices.

Rashi there says that the shem rabba refers to the 42 letter name of Hashem which he calls the shem hameforash. Rashi seems to learn the gemara in Kiddushin 71a that the 42 letter name mentioned there was the shem hameforash - against the shittas HaRambam. So the question is how does the Rambam learn the gemara in Sanhedrin? What is the Shem Rabba according to the Rambam.

The Rambam in Hilchos Avodah Zarah 2:7 provides an answer:

יב אֵין הַמְּגַדֵּף חַיָּב סְקֵלָה, עַד שֶׁיְּפָרַשׁ אֶת הַשֵּׁם הַמְּיֻחָד שֶׁלְּאַרְבַּע אוֹתִיּוֹת שְׁהוּא אֶלֶף דֶּלֶת נוּן יוֹד, וִיבָרַךְ אוֹתוֹ בְּשֵׁם מִן הַשֵּׁמוֹת שְׁאֵינָן נִמְחָקִין--שֶׁנֶּאֱמָר "וְנֹקֵב שֵׁם-ה' מוֹת יוּמָת" (ויקרא כד,טז), עַל הַשֵּׁם הַמְּיֻחָד חַיָּב סְקֵלָה; וְעַל שְׁאָר הַכִּנּוּיִין, בְּאַזְהָרָה. וְיֵשׁ מִי שֶׁמְּפָרֵשׁ שְׁאֵינוּ חַיָּב אֵלָא עַל שֵׁם יוֹד הֵא וָאו הֵא; וַאֲנִי אוֹמֵר שֶׁעַל שְׁנֵיהֶם, הוּא נִסְקָל

The Rambam here states his opinion that by megadef one is chayav for both four letter names of Hashem, whether it be alef daled or whether it be yud key. Where does he get this halacha?

Apparently, he learns the gemara in Sanhedrin that the four letter name there was the name of Alef Daled. The gemara said that he chiddush is that you don't require the "shem rabba" - the yud key shem hameforash to be chayav - but rather either four letter name of Hashem suffices. Thus, the Rambam's chiddush that even the shem alef daled is chayav by megadef is a direct consequence of his shittah that the shem hameforash is yud key vav key.

[As a final note - in case you are wondering what happened to the well known 72 letter name of Hashem - see Breishis Rabbah that quotes R' Avina that Hashem redeems His children with his 72 letter name.]

*See the Tzafnas Paneach in Hilchos Avodah Zarah that seems to be indicating this pshat in the Rambam.