Seaweed is a plant-like living thing that grows in the sea, often on rocks or floating in the water. English stuck sea and weed together. Weed here is the old English plant-word (Old English wēod), not only the “unwanted plant in the garden” meaning people learn first — though actually, that garden meaning grew out of the same old word. Many kinds of seaweed are useful. People eat some of them, put them in soup, or wrap food in thin sheets of them. In shops you may hear more exact food names, but seaweed is the everyday general word for the living thing in the water.

海藻。吃的那種，台灣更常說海苔、紫菜、海帶，比 seaweed 更具體。