Definition of alga
: a plant or plantlike organism of any of several phyla, divisions, or classes of chiefly aquatic usually chlorophyll-containing nonvascular organisms of polyphyletic origin that usually include the green, yellow-green, brown, and red algae in the eukaryotes and especially formerly the cyanobacteria in the prokaryotes
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Merriam-Webster unabridged
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Time Traveler for alga
The first known use of alga was in 1551
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