Wikipedia definition [as of 3/28/2011]: In The Church of Jesus Christ of Latter-day Saints, a hosanna shout is an organized ritual by a congregation of shouting hosanna. It was first performed as a ritual in the dedication of the Kirtland Temple, and was a part of the Kirtland Endowment ceremony. It continues to be practiced by some Latter Day Saint denominations, most notably The Church of Jesus Christ of Latter-day Saints, which practices the ritual at the dedication of each of its temples.
The sacred ceremony of the Hosanna Shout is practiced by members of the Church of Jesus Christ of Latter-day Saints. It is usually reserved for temple dedications. It is given in the spirit of thanksgiving and petition, fulfilling the instruction to bless the name of the Lord with loud voices and "with a sound of rejoicing", with "hosannas to him that sitteth upon the throne forever" (D&C 19:37; 36:3; 39:19; 124:101).
When the ordinance of the washing of feet was introduced at Kirtland, shouts of hosanna were viewed as a sealing benediction on both private and quorum prayer and then on the dedicatory prayer. At prayer meetings in the Kirtland Temple, the Saints sometimes used related phrases such as "Blessed is the name of the Most High God" and "Glory to God in the highest" (HC 2:386).
The Hosanna Shout is whole-souled, given to the full limit of one's strength. The congregation stands and in unison shouts the words "Hosanna, Hosanna, Hosanna to God and the Lamb. Amen, Amen, and Amen," repeating the phrase three times. This is done while waving of a white handkerchiefs with uplifted hands. The epithet "Lamb" relates to the condescension and Atonement of Jesus Christ.
The Hosanna Shout memorializes the pre-earthly Council in Heaven, as "when…all the sons of God shouted for joy" (Job 38:7). It also recalls the hosannas and the waving of palm branches accorded the Messiah as he entered Jerusalem. And hosannas welcomed him as he appeared to the Nephites. President Lorenzo Snow, the fifth prophet of the restored church, taught that this shout will herald the Messiah when he comes in the glory of the Father (cf. 1 Thes. 4:16).