Rille is a geological term used to describe a long, narrow and shallow depression or trench found on the surface of the moon. Synonyms for this term include groove, channel, furrow, crease and gutter. A rille can be caused by various events such as volcanic activity, impact or erosion. These depressions can be found all over the moon's surface and can range in size from a few meters up to hundreds of kilometers long. Other synonyms for rille can include gully, ravine, or canyon, which describe larger, deeper and more defined depressions on the surface of the moon or planets.