Judaism
YHWH ("Yehovah"/"Yahweh") is the most sacred name of God in Hebrew, and is not pronounced by most Jews.  No one knows where the name came from, or what exactly it means.  It looks like the Hebrew word "hayah," which is the verb "to be." (According to Hebrew scripture, when Moses asked God who God was, God told Moses I am that I am/I am who I am.) Jews believe that the name YHWH shows that God is endless.  Instead of trying to say it, most Jews say "haShem", which means "The Name."  Some people pronounce this name as Yahweh, or Jehovah.  Scholars of religion sometimes refer to "YHWH" as the Tetragrammaton, from an old Greek word that means "four letters".