Catholic Encyclopedia (1913)/Adonai
Adonai (אדני), lord, ruler, is a name bestowed upon God in the Old Testament. It is retained in the Vulgate and its dependent versions, Exod., vi, 3; Judith, xvi, 16. No other name applied to God is more definite and more easily understood than this. Etymologically it is the plural of Adon, with the suffix of the possessive pronoun, first person, singular number. This plural has been subjected to various explanations. It may be looked upon as a plurale abstractum, and as such it would indicate the fullness of divine sway and point to God as the Lord of lords. This explanation has the endorsement of Hebrew grammarians, who distinguish a plurale virium, or virtutum. Others prefer to designate this form as plurale excellentiæ, magnitudinis, or plurale majestatis. To look upon it as a form of politeness such as the German Sie for du, or French volts for to is certainly not warranted by Hebrew usage. The possessive pronoun has no more significance in this word than it has in Rabbi (my master), Monsieur, or Madonna. Adonai is also the perpetual substitute for the ineffable Name Yahve, to which it lends its vowel signs. Whenever therefore, the word Yahve occurs in the text, the Jew will read Adonai.
Kautzsch-Gesenius, Hebræische Grammatik (Leipzig, 1896); Dalman, Der Gottesname and seine Geschichte (Berlin 1889); Stade, Biblische Theologie des Alten Testaments (Tübingen, 1905).