The Collaborative International Dictionary of English v.0.48:

Brookweed \Brook"weed`\, n. (Bot.)
   A small white-flowered herb (Samolus Valerandi) found
   usually in wet places; water pimpernel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brookweed
    n 1: American water pimpernel [syn: brookweed, Samolus
         parviflorus, Samolus floribundus]
    2: water pimpernel of Europe to China [syn: brookweed,
       Samolus valerandii]