seaweed
n.plants that grow in the sea or on rocks by the ocean. You can find it on the beach, and some types are used in cooking.
n.large algae growing in the sea or on rocks below the high-water mark. Often used as a collective term for various marine macroalgae.
“The beach was covered in green seaweed after the storm.”
“Many coastal cultures harvest seaweed to use as a nutrient-rich ingredient in soups and salads.”
“Marine biologists are studying how certain species of seaweed can sequester carbon dioxide more efficiently than land-based forests, potentially mitigating the effects of climate change.”
Uncountable when referring to the plant material in general; countable when referring to specific biological species.
From sea + weed.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.