The following is reprinted with permission from A Short Introduction to Judaism , published by Oneworld Publications.
The Jewish God is not merely a philosophical concept, a final cause which explains the existence of the universe. He is a personal God, the true hero of the biblical stories, and the guide and mentor of His Chosen People. As such He has a proper name. In the Hebrew scriptures that name is written as JHWH, since Hebrew script originally contained no vowels. God’s name was almost certainly pronounced in early times, but by the third century BCE the consonants were regarded as so sacred that they were never articulated. Instead, the convention was to read the letters as Adonai, which means “Lord.” Thus in English translations of the Hebrew text, JHWH is never written as a proper name, but as “the Lord.”
JHWH is explained in the book of Exodus as “I am Who I am” and it is clearly derived from the old Hebrew verb HWH which means “to be.” The term “Jehovah” was introduced by Christian scholars. It is merely JHWH pronounced with the vowel of Adonai, thus making JeHoWaH. It is a hybrid and is not usually used by Jews. Over the course of time, even the title Adonai was regarded as too awesome to represent the four letters of God’s name and today most Orthodox Jews use [the term] HaShem, which simply means “the Name.”
Terms for God are treated with the greatest reverence. Among the strictly traditional, even English translations are perceived as too holy to write and today the custom is to inscribe G‑d, the L‑rd and even the Alm‑ghty. This carefulness is explained and justified by the prohibition in the Ten Commandments: “You shall not take the name of JHWH your God in vain; for JHWH will not hold him guiltless who takes His name in vain” (Exodus 20:7).
In ancient times the term Adonai was not just used for God. It was a common mode of address to kings, slave‑masters, and even by wives to husbands. The “i” at the end signifies “my” and, in fact, Adonai is a plural form so it literally means “my lords.” In many verses of scripture and in the liturgy, God is spoken of as JHWH (pronounced Adonai) Eloheynu, which means “the Lord our God.”