The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Tetragrammaton
    n 1: 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