A long, well-defined narrow furrow on a planetary surface; pl. rimae. The name, which means crack or fissure, was first used for features on the Moon in the 17th century. It is applied to features of various origins. For example, Rima Hadley (Hadley Rille) on the Moon is a sinuous rille, formed as a lava channel in a vast lava flow, whereas the Rimae Hippalus are graben surrounding a large impact basin.