Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant since it is not made of more than one clearly differentiated tissue; it is a heterokont.
Kelp grows in "underwater forests" in shallow oceans, and is thought to have appeared in the Miocene, 5 to 23 million years ago. The organisms require nutrient-rich water with temperatures between 6 and 14 °C. They are known for their high growth rate—the genera Macrocystis and Nereocystis can grow as fast as half a metre a day, ultimately reaching 30 to 80 metres.
Through the 19th century, the word "kelp" was closely associated with seaweeds that could be burned to obtain soda ash. The seaweeds used included species from both the orders Laminariales and Fucales. The word "kelp" was also used directly to refer to these processed ashes.