a rectangular column that usually projects about a third of its width from the wall to which it is attached
source: WordNet 3.0
An upright architectural member right-angled in plan, constructionally a pier (See Pier, 1 (b)), but architecturally corresponding to a column, having capital, shaft, and base to agree with those of the columns of the same order. In most cases the projection from the wall is one third of its width, or less.
