Guinevere Planitia is an expansive lowland region of Venus that lies east of Regio and west of Eistla Regio These low-lying plains, particularly in the western portion, are characterized by apparent volcanic source vents and broad regions of bright, dark, and mottled deposits.They are the only break in an equatorially connected zone of highlands and tectonic zones. The types, numbers, and patterns of mapped tectonic features and small volcanic landforms in the region provide important detail in the interpretation and evolution of venusian landscape. By using Pioneer Venus, Goldstone, and Arecibo data, these regions have been interpreted as widespread superposed flows that originated from the source vents. (wikipedia entry)
A planitia is a low plain. On Venus they are named after mythological heroines.
Image Source. Marker locations are approximate and may not always appear to align with image on map.