The red algae, or Rhodophyta ( or ; from Ancient Greek: ῥόδον rhodon, "rose" and φυτόν phyton, "plant"), are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species; more detailed counts indicate about 4,000 in about 600 genera (3,738 marine species in 546 genera in 10 orders, plus the unclassifiable; and 164 freshwater species in 30 genera in eight orders).