The Tetragrammaton, literally "four letter word" is the proper name of the Judeo-Christian God. In the original Hebrew, the name is spelled Yod-He-Waw-He, but because Hebrew has no vowels, its pronunciation is unknown. Jews traditionally use the word "Adonai" (lord) to avoid speaking the actual name aloud. This is why English translations of the Bible refer to God as "The Lord." There are several English and Latin versions of the Tetragrammaton, YHVH, IHVH and JHVH are the most common. They are unpronounceable as well, though attempts to pronounce JHVH are where we get the term "Jehovah" as a name for God.
As a term for the monotheistic God, the Tetragrammaton has no direct connection to polytheistic Wicca. It does, however, show up in ceremonial magic sometimes, where the four letters correspond to the four elements. It is definitely part of Kabbalah.
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