A colonnade is defined as "a series of columns set at regular intervals and usually supporting the base of a roof structure," though it may also be a free-standing structure. The word comes from the French word for "column" which is, in turn, derived from the Italian word for "column."
The most obvious and recognizable examples of a colonnade can be found in Ancient Greek and Roman architecture. Think of the famous Greek Parthenon, with its many recognizable columns which are still standing, or the many ruins in Rome and other cities in Italy which still have standing columns. More modern examples of colonnades include St. Peter's Basilica in Vatican City and the Lincoln Memorial in Washington, D.C.
The word colonnade can be used for things other than marble or stone columns. For example, a row or trees lining a driveway would also be considered a colonnade.
A colonnade is a row of columns supporting a roof, an entablature or arcade.