Jehovah [B] [H] [S]

the special and significant name (not merely an appellative title such as Lord [adonai]) by which God revealed himself to the ancient Hebrews ( Exodus 6:2Exodus 6:3 ). This name, the Tetragrammaton of the Greeks, was held by the later Jews to be so sacred that it was never pronounced except by the high priest on the great Day of Atonement, when he entered into the most holy place. Whenever this name occurred in the sacred books they pronounced it, as they still do, "Adonai" (i.e., Lord), thus using another word in its stead. The Massorets gave to it the vowel-points appropriate to this word. This Jewish practice was founded on a false interpretation of  Leviticus 24:16 . The meaning of the word appears from  Exodus 3:14 to be "the unchanging, eternal, self-existent God," the "I am that I am," a convenant-keeping God. (Compare Malachi 3:6 ;  Hosea 12:5 ;  Revelation 1:4  Revelation 1:8 .) 

The Hebrew name "Jehovah" is generally translated in the Authorized Version (and the Revised Version has not departed from this rule) by the word LORD printed in small capitals, to distinguish it from the rendering of the Hebrew Adonai and the Greek Kurios , which are also rendered Lord, but printed in the usual type. The Hebrew word is translated "Jehovah" only in  Exodus 6:3 ; Psalms 83:18 ;  Isaiah 12:2 ;  26:4 , and in the compound names mentioned below. 

It is worthy of notice that this name is never used in the LXX., the Samaritan Pentateuch, the Apocrypha, or in the New Testament. It is found, however, on the "Moabite stone" (q.v.), and consequently it must have been in the days of Mesba so commonly pronounced by the Hebrews as to be familiar to their heathen neighbours.