Hallelujah is a word best known from Harlem gospel churches (as in Hallelujah, praise the Lord) and from Handels chorus. Many dont know that its a Hebrew word that appears countless times in the book of Psalms.
Hallelujah is a combination of two words, hallel meaning praise, and the shortened two-letter version of the name of God. For this reason some Jews wont pronounce the word except when reciting a prayer or a biblical verse, but will say hallelukah instead, rather than utter one of Gods names in a profane context.
Hallel, the first part of the word, is also known to shul-goers as the celebratory singing of Psalms 113-118 on most Jewish holidays. Interestingly, we dont say Hallel on Purim, coming up next week. The Talmud (Megillah 14a) suggests three possible explanations for this.
The most challenging reason is that Hallel simply doesnt apply to the Purim story. Why not? Because in the very first line of Hallel, we say: Praise, you servants of God. But at the end of the Purim story, though the Jews have been saved from death, they are still servants of Ahasuerus, subject to his antisemitic whims and caprices. Diaspora life continues as usual.
After the Purim story the Jews still cannot offer the wholehearted praise of servants of God, subject to no other master.