- any of certain long, narrow, straight or sinuous trenches or valleys observed on the surface of the moon.
- a brook or stream; rivulet
- a small channel or gulley, such as one formed during soil erosion
- Also: rille one of many winding cracks on the moon
“small brook, rivulet,” 1530s, from or related to Dutch and Frisian ril, Low German rille “groove, furrow, running stream,” probably from Proto-Germanic *ril- (cf. Old English rið, riþe “brook, stream,” which survives only in dialects), a diminutive form from PIE root *reie- “to run, flow” (see Rhine).