Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Lamina (algae)
The lamina or blade in macroscopic algae like seaweed is a generally flattened structure that typically forms the principal bulk of the thallus. It is often developed into specialised organs such as flotation bladders and reproductive organs.