Arches are semicircular or curved structures that are commonly used in architecture and construction. When referring to arches, there are several synonymous terms that can be used. For example, you can use "vaults" to refer to arches that form a ceiling or roof. "Span" is another term that refers to the horizontal distance that an arch covers. "Curve" is another synonym that can be used when referring to the shape of an arch. Other terms that can be used include "dome", "arc", "lunette", and "intrados". These terms are often used in different contexts to refer to various types of arches or arch-like structures.