Entry 3 senses · 2 variants Webster, 1913 Rill /(rĭl)/ · IPA /ɹɪl/ n. v. i. 01 n. A very small brook; a streamlet. 1. A very small brook; a streamlet. 2. See Rille.(Astron.) See: Rille 02 v. i. To run a small stream. 1. To run a small stream.[R.]