Tetragrammaton is a term of Greek origin, meaning literally, “four letters.” This technical name is given to the four Hebrew letters “JHVH” or “YHVH” of God’s proper name, Yahweh, disclosed by God to Moses out of the burning bush in Exodus 3.
Vowels were added to the four consonants of the name of God to allow vocalization which is believed to approximate the original pronunciation of the word.
About three centuries before Christ, the Jews, out of reverence for the revealed divine Name, Yahweh, ceased pronouncing it.
The Old Testament Hebrew term Adonai, best translated as “Lord” or “my Lord” is attributed to God out of reverence for the majesty and “unspeakably” great name of Yahweh. (8:31)