In The Bible
The term is used 24 times in the Hebrew Bible (mainly in the book of Psalms, e.g. 111–117, 145–150, where it starts and concludes a number of Psalms) and four times in Greek transliteration in the Christian Book of Revelation.
In the Hebrew Bible hallelujah is actually a two-word phrase, not one word. The first part, hallelu, is the second-person imperative masculine plural form of the Hebrew verb hallal. However, "hallelujah" means more than simply "praise Yah", as the word hallel in Hebrew means a joyous praise in song, to boast in God. Hallel could also refer to someone who acts madly or foolishly.
The second part, Yah, is a shortened form of YHWH, the name for the Creator. The name ceased to be pronounced in Second Temple Judaism, by the 3rd century BCE due to religious beliefs. The correct pronunciation is not known, however, it is sometimes rendered by Christians as "Yahweh" or "Jehovah". The Septuagint translates Yah as Kyrios (the LORD), because of the Jewish custom of replacing the sacred name with "Adonai", meaning "the Lord".
In Psalm 150:6 the Hebrew reads kol han'shamah t'hallel yah; It appears in the Hebrew Bible as הללו-יה and הללו יה. In Psalm 148:1 the Hebrew says "הללו יה hallelu yah". It then says "hallelu eth-YHWH" as if using "yah" and "YHWH" interchangeably. The word "Yah" appears by itself as a divine name in poetry about 49 times in the Hebrew Bible (including hallelu yah), such as in Psalm 68:4–5 "who rides upon the deserts by his name Yah" and Exodus 15:2 "Yah is my strength and song". It also often appears at the end of Israelite theophoric names such as Isaiah "yeshayah(u), Yahweh is salvation" and Jeremiah "yirmeyah(u), Yahweh is exalted".
The word hallelujah occurring in the Psalms is therefore a request for a congregation to join in praise toward God. It can be translated as "Praise Yah" or "Praise Jah, you people", and in English translations is usually rendered as "Praise the LORD".
"Hallelujah" appears in Revelation 19 in Greek transliteration as allēlouia (ἀλληλούϊα), the great song of praise to God for his triumph over the Whore of Babylon. This transliteration also appears in the Septuagint.
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