Hallelujah is A transliteration of the Hebrew expression ha·lelu-Yah′, appearing first at Psalm 104:35. It is nearly always translated “praise Jah, you people.” The word means “Praise Jah, you people,” “Jah” being the shortened form of the divine name, Jehovah.
The expression occurs 24 times in the Hebrew Scriptures and, with the exception of Psalm 135:3 “praise Jah", introduces and/or concludes the Psalms in which it is found. (See Psalms 112:1; 115:18; 146:1, 10; 147:1, 20; 148:1, 14; 149:1, 9; 150:1, 6.) Jah is 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 he’ (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.”
In the Christian Greek Scriptures “Jah” appears four times in the expression Hallelujah. (Revelation 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,” or “Praise Jah, you people!”
"Insight on the Scriptures," Bible Encyl. Vol 1