Arch end is a term often used in architecture to refer to the underside of an arch or vault. However, there are several synonyms for this term that are equally valid and commonly used. One of the most popular synonyms for arch end is "keystone," which refers to the central stone at the top of an arch. Other synonyms for arch end include "voussoir," which is a wedge-shaped stone used to form an arch, and "springer," which refers to the stone where the arch begins to rise from the ground. Regardless of the specific term used, all of these synonyms for arch end are important elements in creating strong and stable architectural structures.