The Collaborative International Dictionary of English v.0.48:

Pillwort \Pill"wort`\, n. (Bot.)
   Any plant of the genus Pilularia; minute aquatic
   cryptograms, with small pill-shaped fruit; -- sometimes
   called peppergrass.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pillwort
    n 1: European water fern found around margins of bodies of water
         or in wet acid soil having small globose sporocarps [syn:
         pillwort, Pilularia globulifera]