The Collaborative International Dictionary of English v.0.48:

Portico \Por"ti*co\, n.; pl. Porticoesor Porticos. [It., L.
   porticus. See Porch.] (Arch.)
   A colonnade or covered ambulatory, especially in classical
   styles of architecture; usually, a colonnade at the entrance
   of a building.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Colonnade \Col`on*nade"\, n. [F. colonnade, It. colonnata, fr.
   colonna column. See Colonel.] (Arch.)
   A series or range of columns placed at regular intervals with
   all the adjuncts, as entablature, stylobate, roof, etc.
   [1913 Webster]

   Note: When in front of a building, it is called a portico;
         when surrounding a building or an open court or square,
         a peristyle.
         [1913 Webster]

WordNet (r) 3.0 (2006):

portico
    n 1: a porch or entrance to a building consisting of a covered
         and often columned area

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "portico":
   access, aisle, alley, ambulatory, aperture, arcade, artery, atlas,
   avenue, caryatid, channel, cloister, colonnade, colonnette, column,
   communication, conduit, connection, corridor, covered way, defile,
   entrance, entrance hall, entranceway, entry, entryway, exit, ferry,
   ford, foyer, galilee, gallery, inlet, interchange, intersection,
   junction, lane, lobby, narthex, opening, outlet, overpass, pass,
   passage, passageway, peristyle, pier, pilaster, pillar, porch,
   portal, post, propylaeum, railroad tunnel, stoa, telamon,
   threshold, traject, trajet, tunnel, underpass, veranda,
   vestibule