Source : Webster's Revised Unabridged Dictionary (1913)

Seaweed \Sea"weed`\, n.
   1. Popularly, any plant or plants growing in the sea.

   2. (Bot.) Any marine plant of the class Alg[ae], as kelp,
      dulse, Fucus, Ulva, etc.

Source : WordNet®

seaweed
     n : plant growing in the sea, especially marine algae