plural noun sing. alga
any of several divisions of simple photosynthetic organisms, esp. certain thallophytes, variously one-celled, colonial, or filamentous, containing chlorophyll and other pigments (esp. red and brown), and having no true root, stem, or leaf: algae are found in water or damp places, and include seaweeds and pond scum
Origin:
pl. of L alga, seaweed < IE base *el-, *ol-, to be moldy, putrid > Swed ul, rancid, Du uilig, rotten