Wiktionary
n. A small moon that orbits near the outer edges of a planetary ring or within gaps in the ring. The edge of the ring remains sharply defined owing to the gravity of shepherd moons.
Wikipedia
A shepherd moon (also herder moon, watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particles as a shepherd.
Due to their gravitational effect, they pick up particles and deflect them from their original orbits through orbital resonance. This causes gaps in the ring system, such as the particularly striking Cassini Division, as well as other characteristic bands, or strange "twisted" deformation of rings.
Usage examples of "shepherd moon".
Before her, just past the short horizon of the shepherd moon, Saturn was an immense wall across the sky.
Where Pham's beam had hit, a corona of light was dimming from blue through orange and red, its light still brighter than the shepherd moon overhead.
Previously scattered clouds, as woolly as sheep, have been herded together around the shepherd moon.