The Collaborative International Dictionary of English v.0.48:

Alga \Al"ga\, n.; pl. Alg[ae] or algae. [L., seaweed.]
   (Bot.)
   A kind of seaweed; pl. the class of cellular cryptogamic
   plants which includes the black, red, and green seaweeds, as
   kelp, dulse, sea lettuce, also marine and fresh water
   conferv[ae], etc. The algae are primitive
   chlorophyll-containing mainly aquatic eukaryotic organisms
   lacking true stems and roots and leaves.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

alga
    n 1: primitive chlorophyll-containing mainly aquatic eukaryotic
         organisms lacking true stems and roots and leaves [syn:
         alga, algae]