Alga \Al"ga\, n.; pl. Alg[ae]. [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.

Source: Webster's Revised Unabridged Dictionary (1913)