Source : Webster's Revised Unabridged Dictionary (1913)

Tetragrammaton \Tet`ra*gram"ma*ton\, n. [NL., fr. Gr. ?; te`tra-
   (see {Tetra-}) + ? a letter.]
   The mystic number four, which was often symbolized to
   represent the Deity, whose name was expressed by four letters
   among some ancient nations; as, the Hebrew JeHoVaH, Greek
   qeo`s, Latin deus, etc.

Source : WordNet®

Tetragrammaton
     n : four Hebrew letters usually transliterated as YHWH (Yahweh)
         or JHVH (Jehovah) signifying the Hebrew name for God
         which the Jews regarded as too holy to pronounce