The Collaborative International Dictionary of English v.0.48:

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
   (Anagallis arvensis) has small flowers, usually scarlet,
   but sometimes purple, blue, or white, which speedily close at
   the approach of bad weather.
   [1913 Webster]

   Water pimpernel. (Bot.) See Brookweed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water pimpernel \Wa"ter pim"per*nel\ (Bot.)
   A small white-flowered shrub; brookweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water pimpernel
    n 1: a white-flowered aquatic plant of the genus Samolus