Oblong enclosures curved at one end and built for horse and chariot racing, usage is typically restricted to such structures in Greek cities of the Hellenistic, Roman, and Byzantine eras; meaning overlaps with similar structures in ancient Rome itself, "circuses (Roman arenas)." For sports structures used for horse racing in more modern contexts, use "horse racetracks."

Source: AAT

http://vocab.getty.edu/page/aat/300108449