Source : Webster's Revised Unabridged Dictionary (1913)

Quoin \Quoin\, n. [See {Coin}, and cf. {Coigne}.]
   1. (Arch.) Originally, a solid exterior angle, as of a
      building; now, commonly, one of the selected pieces of
      material by which the corner is marked.

   Note: In stone, the quoins consist of blocks larger than
         those used in the rest of the building, and cut to
         dimension. In brickwork, quoins consist of groups or
         masses of brick laid together, and in a certain
         imitation of quoins of stone.

Source : WordNet®

quoin
     n 1: expandable metal or wooden wedge used by printers to lock up
          a form within a chase [syn: {coign}, {coigne}]
     2: the keystone of an arch [syn: {coign}, {coigne}]
     3: (architecture) solid exterior angle of a building;
        especially one formed by a cornerstone [syn: {corner}]