A pilaster, as used in architecture, is a slightly-projecting column built into or onto a wall. A pilaster is a flattened or abbreviated column that can appear with a capital and entablature, also in "low-relief" or flat against the wall. The pilaster is an architectural element in classical architecture and is used to give the appearance of a supporting column, with only an ornamental function. In contrast, an engaged column or buttress can support the structure of a wall and roof above.
Pilasters often appear on the sides of a door or window opening on the facade of a building, and are sometimes paired with square or round columns set at some distance away from the wall that support a roof structure above, such as a portico. These vertical elements can also be used to support a recessed archivolt around a doorway. The pilaster can be replaced by ornamental brackets supporting the entablature or a balcony over a doorway.
A pilaster can have a simple rectangular profile (cross section) or can be ornamented in one of the styles of the classical orders, appearing in the giant order as two-story tall, fluted columns. The fashion of using this element from Ancient Greek and Roman architecture was adopted in the Italian Renaissance and later Greek Revival architecture.