'Geoglyph' is a word occasionally used in anthropology and archeology to refer to large lithic artifacts that are intended to remain permanently, and which generally take the form of recognizable symbolic or geometric designs. The term is most often used when referring to the Nazca Lines and surrounding designs (which may also be referred to as intaglios), but may also be used to refer to henges (including Stonehenge), stone circles, and chalk figures (such as the Long Man of Wilmington). As such, the term excludes naturally occurring geological features.
A less technical description might be "any large example of rock art". The term 'geoglyph' is starting to come into use to refer to modern earthwork art as well, referring to projects such as Land art (including the famous Spiral Jetty), Earthworks, and Britglyph.