(plural), 1794, from alga (singular), 1550s, from Latin alga "seaweed," which is of uncertain origin. Perhaps connected to Latin ulva "grass-like or rush-like aquatic plant," or perhaps from a PIE root meaning "to putrefy, rot," but de Vaan considers this unlikely and suggests it might be a foreign loan-word.
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Definitions of alga from WordNet
primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves;