It seems you are referring to the Roman arch, which indeed reached its pinnacle in Roman architecture. The arch originated with the Etruscans and was further developed by the Romans, enabling the construction of large and stable structures.
The term you're looking for that describes the projection from an arch to support an entablature is known as the "impediment" or "architrave," which refers to the horizontal structure that rests on the columns or arches that support it, forming an essential component of classical architecture.
If you need additional information or details on Roman architecture or specific structures that exemplify these principles, feel free to ask!