A baldacchino is an architectural canopy above the main altar in a church. The term may also derivatively be used of a canopy over the cathedra or episcopal throne, also over a statue or shrine. It also means the portable cloth canopy held over the Blessed Sacrament in Eucharistic processions, and this is the original meaning in religious contexts since the term refers strictly to a type of cloth.
The term originated in Italy, and (bizarrely) is a corruption of the name of the city of Baghdad. In the later Middle Ages this place was the source of a high-value heavily embroidered brocade cloth which was used for bedspreads, wall-hangings and canopies. Very important people who were involved in processions were dignified by having a sun-shade made of this cloth, and held on four poles over them. In this way, the term mutated to mean a stone canopy over a free-standing altar, supported by four columns.
This architectural feature is referred to as a ciborium, since it shelters the Blessed Sacrament on the altar during Mass, but this usage does lead to confusion with the liturgical vessel called a ciborium where the term comes from. The term ciborium magnum ("great ciborium") may also be used, and is more clear. The modern Italian use of ciborio to refer to the tabernacle is mistaken.
In English, a baldacchino is rendered as "baldachin" or "baldaquin"; the first derives directly from the Italian, and the second is mediated through the French.