A canopy over an altar, tomb, or throne. The original meaning of ‘baldachin’ is a silk cloth from Baghdad (baldacco in Italian). A baldacchino is supported by columns and can be portable or fixed, the most famous example of the latter being Bernini's great structure, built 1624–33, for the interior of St Peter's, Rome and placed, as a symbol of the enduring power of the Catholic Church, over the tomb of Christ's earthly successor, St Peter. See ciborium.
http://www.saintpetersbasilica.org/Altars/PapalAltar/PapalAltar.htm Description and illustration of Baldacchino on St Peter's website.