The Collaborative International Dictionary of English v.0.48:

Pier \Pier\, n. [OE. pere, OF. piere a stone, F. pierre, fr. L.
   petra, Gr. ?. Cf. Petrify.]
   1. (Arch.)
      (a) Any detached mass of masonry, whether insulated or
          supporting one side of an arch or lintel, as of a
          bridge; the piece of wall between two openings.
      (b) Any additional or auxiliary mass of masonry used to
          stiffen a wall. See Buttress.
          [1913 Webster]

   2. A projecting wharf or landing place.
      [1913 Webster]

   Abutment pier, the pier of a bridge next the shore; a pier
      which by its strength and stability resists the thrust of
      an arch.

   Pier glass, a mirror, of high and narrow shape, to be put
      up between windows.

   Pier table, a table made to stand between windows.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pier
    n 1: a platform built out from the shore into the water and
         supported by piles; provides access to ships and boats
         [syn: pier, wharf, wharfage, dock]
    2: (architecture) a vertical supporting structure (as a portion
       of wall between two doors or windows)
    3: a support for two adjacent bridge spans