What architectural form is a semicircular room or recess known as?

A semicircular room or recess is known as a hemicycle. This term derives from the Greek word "hemicycle," which combines "hemi-" meaning half, and "kyklos," meaning circle. In architectural terminology, a hemicycle typically features a curved or semicircular shape, which can often create a visually pleasing and acoustically advantageous space.

The hemicycle can be found in various architectural contexts, including amphitheaters, which utilize the semicircular form to enhance sound projection and viewing angles. This design often contributes to a sense of cohesion and unity within larger structured spaces, which is particularly significant in historical architecture.

Other terms listed, such as portico, decumanus, and praefectus, refer to different architectural concepts. A portico is a structure consisting of a roof supported by columns at one or more sides, while a decumanus refers to an east-west oriented street in Roman city planning. The term praefectus typically relates to a Roman official rather than an architectural element.