ARCH: A load-bearing device used to span voids in walls or other structures. The arch works by redirecting its load so that the supporting material is under compressive stress instead of shear or tensile stress. Most building materials are many times stronger in compression than in tension or shear, so arches help designers build taller, lighter structures.
Arches are so important, one is tempted to believe that arch is the root of the word architect.
Mere coincidence. Arch is from the Latin arcus – arrow; architect is from the Greek architekton – master builder, according to my Webster’s. That’s altogether appropriate, because the Romans gave us the arch, but the Greeks gave us architecture as a science and profession.
Arches come in many forms, but the most important distinction, from the standpoint of architectural style, is whether the peak of the arch (crown) is curved (Roman, Romanesque, Romanesque Revival) or pointed (Gothic, Gothic Revival). There are also flat arches (having very slightly elevated crowns). Join a series of arches end-to-end and you have a gallery; join a series of arches side-to-side and you have a vault; join a series of arches rotated through 180 degrees and you have a dome.
Here are a few examples from structures in New York City.