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I notice some have used this term in their threads and I want to add to your information about this Biblical term.
A transliteration of the Hebrew expression ha·lelu-Yahʹ, appearing first at Psalm 104:35. It is nearly always translated “praise Jah, you people.” The expression occurs 24 times in the Hebrew Scriptures and introduces and/or concludes the Psalms in which it is found. (See Ps 112:1; 115:18; 146:1, 10; 147:1, 20; 148:1, 14; 149:1, 9; 150:1, 6.). A Greek form of it appears four times at Revelation 19:1-6, where the reference is to the joy experienced over the destruction of Babylon the Great and that associated with Jehovah’s beginning to rule as King.—See JAH.
A poetic shortened form of Jehovah, the name of the Most High God. (Ex 15:1, 2) This abbreviated form is represented by the first half of the Hebrew Tetragrammaton יהוה (YHWH), that is, the letters yohdh (י) and heʼ (ה), the tenth and fifth letters of the Hebrew alphabet respectively.
Jah occurs 50 times in the Hebrew Scriptures, 26 times alone, and 24 times in the expression “Hallelujah,” which is, literally, a command to a number of people to “praise Jah.” However, the presence of “Jah” in the original is completely ignored by certain popular versions. (Dy, Mo, RS) The King James Version and An American Translation have it only once, as “Jah” and “Yah” respectively. (Ps 68:4) In the English Revised Version it appears twice in the body of the text (Ps 68:4; 89:8), and in the American Standard Version the full form, Jehovah, is substituted throughout, but these latter two translations in practically every occurrence of the contracted form call it to our attention in footnotes. The New World Translation preserves for the reader all 50 occurrences of Jah, or Yah; and Rotherham’s Emphasised Bible, 49 of them.
In the Christian Greek Scriptures “Jah” appears four times in the expression Hallelujah. (Re 19:1, 3, 4, 6) Most Bibles simply carry this Greek expression over into English untranslated, but G. W. Wade renders it, “Praise ye Jehovah,” and the New World Translation reads, “Praise Jah, you people!”
The single syllable Jah is usually linked with the more moving emotions of praise and song, prayer and entreaty, and is generally found where the subject theme dwells upon a rejoicing over victory and deliverance, or where there is an acknowledgment of God’s mighty hand and power.
source: "Insight onthe Scriptures" 1988