Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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