Seaweeds are any of a large number of marine plants and protists in the category of benthic algae. They are macroscopic and multicellular, in contrast with most other algae. [1] Seaweeds are often found in the seashore biome.

Seaweeds consist of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups are not thought to have a common multicellular ancestor, the seaweeds are a paraphyletic group. In addition, tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered as seaweeds.

Seaweeds are popularly described as plants, but only red and green algae belong to the kingdom Plantae. They should not be confused with aquatic plants such as seagrasses (which are vascular plants).

Seaweeds' appearance somewhat resembles non-arboreal terrestrial plants.

thallus: the algal body
lamina: a flattened structure that is somewhat leaf-like
sorus: spore cluster
on Fucus, air bladders: float-assist organ (on blade)
on kelp, floats: float-assist organ (between lamina and stipe)
stipe: a stem-like structure, may be absent
holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.
haptera: finger-like extensions of holdfast anchoring to benthic substrate
The stipe and blade are collectively known as fronds.