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