Alga

1. primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves

Any of many aquatic photosynthetic organisms, including the seaweeds, whose size ranges from a single cell to giant kelps and whose biochemistry and forms are very diverse, some being eukaryotic.

Etymology: From Latin alga.

alga

Etymology: Inherited from Latin alga.

seaweed

Etymology: From Latin alga.

alga (any of many aquatic photosynthetic organisms similar to plants or bacteria); seaweed (any marine plant)

Etymology: From Latin alga.

alga (any of many aquatic photosynthetic organisms)

Etymology: Borrowed from German Alge or French algue, from Latin alga. First attested in 1619.

Seaweed; plants that grow in freshwater.; Something of little worth.

Etymology: Of unknown origin, though probably a foreign substrate loanword related to ulva (“various grass-like or rush-like aquatic plants”). Note also similar dubiously-related forms in Baltic and Germanic with connotations of filth and dirtiness, including Lithuanian el̃mės, almens (“ichor, sanies”), (dialectal) Norwegian ul (“moldy”), olga (“to loathe”), elgja (“to fell like vomiting”), ulma (“to get moldy”), Middle Low German olm, ulm (“rottenness (esp. of wood)”), and Dutch uilig (“moldy (of wood)”). These terms have been tentatively linked to a Proto-Indo-European *alg-, *alǵ- (“to be dirty, be slimy; frog; duckweed”); other similar forms include dialectal Norwegian alka (“to dirty, soil”), Norwegian ulk (“frog, slime”), Low German ulk (“frog”). That said, it is possible that the Baltic and Germanic are not related to the Latin, or that all the terms derive from the same substrate.