Source : Webster's Revised Unabridged Dictionary (1913)

Water pimpernel \Wa"ter pim"per*nel\ (Bot.)
   A small white-flowered shrub; brookweed.

Pimpernel \Pim"per*nel\, n. [F. pimprenelle; cf. Sp. pimpinela,
   It. pimpinella; perh. from LL. bipinnella, for bipinnula
   two-winged, equiv. to L. bipennis; bis twice + penna feather,
   wing. Cf. {Pen} a feather.] (Bot.)
   A plant of the genus {Anagallis}, of which one species ({A.
   arvensis}) has small flowers, usually scarlet, but sometimes
   purple, blue, or white, which speedily close at the approach
   of bad weather.

   {Water pimpernel}. (Bot.) See {Brookweed}.

Source : WordNet®

water pimpernel
     n : a white-flowered aquatic plant of the genus Samolus