Quoin, noun

Large, prominent masonry units outlining windows, doorways, segments, and corners.

Another source suggests that the outside corner of a wall is called a quoin, and that the “stones or bricks that form the corner are called stone quoins or quoin bricks.” This is the original definition, and the meaning has since expanded to include distinguishing decoration on a wall. So whether you’re talking about the corner of a building or the architectural decoration on that corner (as well as around windows, doors, and arches), you can impress someone by calling it a quoin.

Word in Use: “There’s a Victorian house near me that had to have its plaster quoins remade. When they took them off, you could see that the people who built the house had used cheaper bricks under the quoins, because they would be hidden under the plaster, and yet the quoins are the image of the cornerstones of a stone building, which are emphasized because they are stronger than the rest!”—Paul Shepheard, What is Architecture?: An Essay on Landscapes, Buildings, and Machines, p. 77.