This is a term applied to the four Hebrew letters that make up the name of God as revealed to Moses in Exodus 3:14
. God said to Moses, "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.’” YHWH makes up the base of the verb "to be" from which God designated His own name as "I AM." In English the letters are basically equivalent to YHWH. It is from these four letters that the name of God is derived and has been rendered as Yahweh and Jehovah. The true pronunciation of God's name has been lost through lack of use, because the Jews, who were first given the name of God, would not pronounce it out of their awe and respect for God.