ADONAI - Superior or Supreme Lord

The word ADONAI is a transliteration of a form of the word ADON as found in the Masoretic Hebrew text. Others have transliterated this form as ADONAY or something similar. ADONAI is built upon the same form that is often transliterated as ADONI (meaning, my lord). The basic form is most often transliterated as ADON (meaning, Lord, Master, Ruler). In the original Hebrew, which was no written vowels, the two forms (ADONI and ADONAI) have the same basic spelling. The difference between the two forms is in the vowel pointing that Masoretes invented centuries after Christ. The Masoretes used a different vowel pointing than the vowel pointing they used for ADONI in every instance in which they believed that the word applied to God, thus forming the word often transliterated as ADONAI. Adding the vowel point to form ADONAI actually makes it plural in form, but in the Masoretic text, ADONAI most often is not used as a plural, but as a "plural intensive" (some refer to this as the "emphatic plural" or "plural of majesty"). Biblical Hebrew many times uses the plural form in a singular setting to intensify the meaning, making the meaning superlative or superior. In the case of application to God, ADONAI would mean Supreme Lord. 

Based on the assumption that the original Hebrew forms are what they call "pictographs", some have claimed that ADONAI means, "The door of life". Actually, the whole idea of giving Hebrew letters meanings as "pictographs" is actually a modern conjecture. There is no indication that the Bible writers ever thought of the Hebrew letters as being what is being labeled as "pictographs". The Bible surely gives no indication that ADONAI means "the door of life".

Some have claimed that in a few instances, the Masoretes were in error, as in Isaiah 6:1, where they added the vowel point to form ADONAI rather than presenting it to as "my lord" (ADONI). Some trinitarians, as well as some others, have claimed that they should have added the vowel point in Psalm 110:1, so as make it say appear that both Jehovah and the one sitting at the right hand of Jehovah are both the Supreme Lord. The only reason, however, we can see for adding the vowel point in Psalm 110:1 would be to make it appear,to be speaking of the added-on trinitarian assumptions.

Some present ADONAI as being one of the "names of God". The form, ADONAI, as actually used in the Masoretic text, is not presented as being a name of God, although one could say it is a "name" in the sense that one could say that "tree" or "man" is a name. Or, one could say it is a "titular" name; however, it is not used as the proper name of God. However, the Jews, having become somewhat superstitious about pronouncing the Holy Name, have often presented a form ADONAI in place of the Holy Name, which, in effect, actually would have them pronouncing (or mispronouncing) the Holy Name as ADONAI (or whatever form they may have used).* Jehovah, however, has never authorized anyone to change his Holy Name to another word as this is often done. 

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*Thus, it is quite probable that some form of ADONAI was spoken, at least in replacing the Holy Name, before the Masoretes performed their work, around 500 AD. 

References: 
ADONAI (Strong's #136), Bible Hub
ADON (Strong's #113), Bible Hub

Entry For Strong's #136 (Studylight)