"Tetragrammaton" literally means "four letters" in Greek. The four letters it refers to are YHWH, which is believed by Jews to be the one true name of God. This is where the name "Yahweh" comes from. The name appears thousands of times in the Hebrew Scriptures but is not to be spoken aloud by Jews. Instead, it is substituted with the titile "Adonai" which means, "the Lord."