The Collaborative International Dictionary of English v.0.48:

Pilaster \Pi*las"ter\, n. [F. pilastre, It. pilastro, LL.
   pilastrum, fr. L. pila a pillar. See Pillar.] (Arch.)
   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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pilaster
    n 1: a rectangular column that usually projects about a third of
         its width from the wall to which it is attached

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "pilaster":
   antenna tower, arcade, atlas, baluster, balustrade, banister,
   barbican, base, belfry, bell tower, campanile, caryatid, colonnade,
   colonnette, colossus, column, cupola, dado, derrick, die, dome,
   fire tower, footstalk, jack, lantern, lighthouse, martello,
   martello tower, mast, minaret, monument, newel-post, obelisk,
   observation tower, pagoda, pedestal, pedicel, peduncle, peristyle,
   pier, pile, piling, pillar, pinnacle, plinth, pole, portico, post,
   pylon, pyramid, queen-post, shaft, skyscraper, socle, spire, staff,
   stalk, stanchion, stand, standard, standpipe, steeple, stem, stupa,
   subbase, surbase, telamon, television mast, tope, tour, tower,
   trunk, turret, upright, water tower, windmill tower