1. In structures, a relatively long, slender structural compression member such as a post, pillar, or strut; usually vertical, supporting a load which acts in (or near) the direction of its longitudinal axis. 2. In classical architecture, a cylindrical support consisting of a base (except in Greek Doric), shaft, and capital; either monolithic or built up of drums the full diameter of the shaft. 3. A pillar standing alone as a monument. (