The divine name in Hebrew — YHWH, the Tetragrammaton — was considered so sacred that observant Jews avoided pronouncing it in ordinary speech, substituting Adonai (Lord) instead. The KJV follows this convention: wherever YHWH appears in the Hebrew text, the KJV prints LORD in small caps. But Yah, the shortened form used in poetry and compounds, appears in contexts where the full name might not, and it runs through the Psalms constantly. Hallelu-Yah is praise directed specifically at the God whose name is Yah.
The word hallal, from which hallelu comes, means to shine, to boast, to make a show of — it is exuberant, loud, public declaration. When combined with the divine name, Hallelujah is not a quiet interior acknowledgment but a command (plural imperative: all of you, praise) to make public noise about the God whose name is Yah. The Hallel Psalms — 113 through 118 — were sung at Passover and form the framework around which Jesus and his disciples ate the Last Supper. When Matthew 26:30 says "And when they had sung an hymn, they went out," they almost certainly had just sung the concluding Hallel psalms, including the last verses of Psalm 118. Jesus went from singing Hallelujah to Gethsemane.
The word survived intact into Greek (allelouia in the Septuagint), Latin (alleluia in the Vulgate), and eventually every language that received the Christian scriptures. No translator of any era in any language ever replaced it with a local word. The word simply arrived and stayed. Revelation 19:1–6 is the only place in the entire New Testament where the word appears — four times in rapid succession — in the scene of final cosmic praise following the judgment of Babylon.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.