/ˌhælɪˈluːjə/
From the Hebrew phrase halləlū-Yāh (הַלְלוּ־יָהּ), which is a compound of halləlū ("praise ye") and Yāh (a shortened form of YHWH, the personal name of God). It literally means "Praise the LORD!"
Hallelujah is an exclamatory command to a group to join in praising God. It is a pure, unadulterated expression of worship, joy, and adoration directed toward the Lord for who He is and what He has done. It appears frequently in the Psalms, especially the "Hallel" psalms (113-118 and 146-150), and culminates in the great chorus of heaven in Revelation 19 celebrating God's ultimate victory.
HALLELU'JAH, n. [Heb. praise ye Jehovah.] Praise ye Jehovah. A word used in hymns and anthems, as an expression of praise and rejoicing.
The word has been largely secularized and is often used as a generic exclamation of joy, relief, or triumph, completely detached from its sacred meaning of praising God. It might be shouted at a concert or upon finding lost keys, stripping the term of its specific, God-ward direction and profound theological weight.