Webster's English Dictionary

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rill

1. rill \'ril\ n [D ril or LG rille; akin to OE ri-th rivulet] : a very 
   small brook 
2. rill vi : to flow like a rill 
3. rill or rille \'ril\ \'ril, 'ril-*\ n [G rille, lit., channel made by a 
   small stream, fr. LG, rill] : any of several long narrow valleys on the 
   moon's surface