"Seaweed" lacks a formal definition, but seaweed generally lives in the ocean and is visible to the naked eye. The term refers to both flowering plants submerged in the ocean, like eelgrass, as well as larger marine algae. Generally, it is one of several groups of multicellular algae: red, green and brown. They lack one common multicellular ancestor, forming a polyphyletic group. In addition, bluegreen algae (Cyanobacteria) are occasionally considered in seaweed literature.
