Let’s break it down, seaweeds are found in the sea and are weeds? That statement is about half right; they are found in the sea and are can occur on almost every sea shore, sometimes you can find them in deeper water where light can just about reach, but it doesn’t make them weeds.
Scientists call seaweeds macroalgae (or macrophytes), “macro” means large and “algae” are simple plants. Algae can be found in the sea, on the land and in freshwater. They are complex at a microscopic level and can be single-celled or multicellular (many cells).
Algae are not like normal plants, they don’t have roots, leaves or stems but they do contain chlorophyll which allows them to change sunlight to energy (photosynthesis). During the day, algae produce oxygen and consume it at night, exactly like terrestrial plants and trees.
My definition of a seaweed
“Seaweeds are simple plants that live in the sea, they photosynthesise and form an important productive part of the marine ecosystem.”