The first is the Covenant Name of Hashem (YHWH), which is traditionally not pronounced. Hundreds of years ago, the Masoretes gave us “ketiv qere” indicators to tell us what words to substitute for the Covenant Name (aka “tetragrammaton“). “Ketiv qere” means “written (differently than) read.”
When the niqqudot (vowel points) from the title “Adonai” (L-RD) are placed under the Name, it is traditionally read “Adonai“ instead of attempting to pronounce YHWH and guessing at what vowels go there; and when the niqqudot from the title “Elohim” (G-D) are present, we substitute “Elohim“ for the tetragrammaton. We do this out of reverence for the Holy One, blessed be He, and in obedience to the 3rd Commandment of the Decalogue: “You are not to take up the name of YHWH your God for emptiness, for YHWH will not clear him that takes up His name for emptiness” (Shemot/Exodus 20:7 TSB*). For another nuance to this discussion, see Prof. Brian Tice’s article on the historical use of nominae sacrae.
In many of our prayers, we here the juxtaposition of two titles of Hashem: “Adonai Eloheinu” (L-RD our G-D). It has been observed that in their contextual usage, the tetragrammaton appears in Scripture when the 13 attributes of mercy are being emphasized (grace, mercy, compassion, etc.), and the title Elohim (the lexical form from which Eloheinu comes) is used when the attributes of Judge and Creator are in view. When both are used in conjunction, both sets of attributes are in view.**
*TSB = Everett Fox, The Schocken Bible: The Five Books of Moses (New York: Schocken Books, 1995; used with permission). This translation was chosen for its diligence in preserving not only the literal meaning of the Hebrew text, but also the original rhythm of the text. It is a masterful translation, and highly recommended.
Grammar note: The shoresh (root) of this word is נתן (to give as a gift). The first nun is absent from this form because of assimilation — i.e. that, being a weak letter, it has transformed into a dagesh (doubling dot) in the following letter (taw) because of the addition of the mem prefix. This is common in Hebrew. Third-position heys tend to apocopate (disappear), and first-position nuns like to assimilate.
Just before the Israelites escaped from their cruel Egyptian taskmasters in 1447 BCE, they were given instructions to paint lamb blood on their doorframes using אזוב (ezov).
וּלְקַחְתֶּ֞ם אֲגֻדַּ֣ת אֵזֹ֗וב וּטְבַלְתֶּם֮ בַּדָּ֣ם אֲשֶׁר־בַּסַּף֒ וְהִגַּעְתֶּ֤ם אֶל־הַמַּשְׁקֹוף֙ וְאֶל־שְׁתֵּ֣י הַמְּזוּזֹ֔ת מִן־הַדָּ֖ם אֲשֶׁ֣ר בַּסָּ֑ף וְאַתֶּ֗ם לֹ֥א תֵצְא֛וּ אִ֥ישׁ מִפֶּֽתַח־בֵּיתֹ֖ו עַד־בֹּֽקֶר׃
The Greek Septuagint renders the word אזוב as ὕσσωπος (hyssop)… but the Septuagint is notorious for its numerous errors.
A number of texts in Ancient Babylonian Aramaic survive from when Daniel was the head of the Magi there, including one which translates a portion of Exodus. What Aramaic word is used to render אזוב? It is a word with which you may be familiar if you know Middle Eastern cooking. The Aramaic word is צעאתאר-יהודי, i.e. Israeli oregano. This word came to be adopted into Hebrew by 536 BCE and was later truncated to צעאתאר (za’atar).
Many centuries later, when the Arab language developed from a dialect of Aramaic to a language of its own, it also adopted the same Aramaic word already in use in Hebrew. Arabs more often use this word to connote a blend of spices containing oregano, marjoram, sumac, and thyme… but the Hebrew usage continues to preserve the original meaning shared with אזוב.