Source : Webster's Revised Unabridged Dictionary (1913)

Fireweed \Fire"weed`\, n. (Bot.)
   (a) An American plant ({Erechthites hiercifolia}), very
       troublesome in spots where brushwood has been burned.
   (b) The great willow-herb ({Epilobium spicatum}).

Source : WordNet®

fireweed
     n 1: tall North American perennial with creeping rootstocks and
          narrow leaves and spikes of pinkish-purple flowers
          occurring in great abundance in burned-over areas or
          recent clearings; an important honey plant [syn: {giant
          willowherb}, {rosebay willowherb}, {wickup}, {Epilobium
          angustifolium}]
     2: an American weedy plant with small white or greenish flowers
        [syn: {Erechtites hieracifolia}]