(pl. regionis, regiones)
A term applied to any feature on a planetary surface that is not clearly defined or understood, usually because of insufficient resolution. Examples include dark regions on Ganymede and Iapetus. On Venus, the term was originally used for radar-bright features such as Beta Regio, which is probably a volcanic construct. Its usage is now extended to cover elevated terrain smaller than continents.
Subjects: Earth Sciences and Geography.