The four-letter name of God, in JudeoChristian lore.  Four Hebrew letters(Tetra, "four";  "Gramma"-something, "letter") without vowels between them, usually Romanized "YHVH" or similarly.  Pronunciation of this name is a BadThing for Hebrews, and its intonation is forbidden for them.  Popular English/Christian guesses at the vowels include "Jehovah" and "Yahweh"(Y and J, and V and W, are represented by the same letter in Hebrew--I think).

The Universal Jewish Encyclopedia, Volume 9, page 160, confirms this fact.
Of the names of God in the Old Testament, that which occurs most frequently (6,823 times) is the so-called Tetragrammaton, YHWH (hwhy), the distinctive personal name of the God of Israel.

The Jewish Encyclopedia of 1901, Volume 12, page 119, states.

It thus becomes possible to determine with a fair degree of certainty the historical pronunciation of the Tetragrammaton, the results agreeing with the statement of Ex. iii. 14, in which YHWH terms Himself  hyha. "I will be", a phrase which is immediately proceeded by the fuller term "I will be that I will be," or, as in the English versions, "I am" and "I am that I am." The name  hwhy is accordingly derived from the root  hwh(=hyh), and is regarded as an imperfect. This passage is decisive for the pronunciation "Yahweh"; for the etymology was undoubtedly based on the known word.