Adonai

Adonai is a Hebrew word for "Lord" used by the Jews as a substitute for the Tetragrammaton, YWHH, which is the name of God (YHWH = I AM) as revealed in Exodus 3:14, "And God said to Moses, “I AM WHO I AM”; and He said, “Thus you shall say to the sons of Israel, ‘I AM has sent me to you.’”

The name of God, YHWH, was so reveared by the Jews that when reading and writing the name of God the Jews would substitute the four letters for 'adonai' so as not to accidentally use the Lord's name in vain per Exodus 20:7.