A fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as polypodiophyta, or polypodiopsida when treated as a subdivision of tracheophyta (vascular plants). The study of ferns is called pteridology; one who studies ferns is called a pteridologist.

The term pteridophytes has traditionally been used to describe all seedless vascular plants so is synonymous with "ferns andfern allies". This can be confusing given that the fern phylum Pteridophyta is also sometimes referred to as pteridophytes.Afern is a vascular plant that differs from the more primitive lycophytes in having true leaves (megaphylls), and from the more advanced seed plants (gymnosperms and angiosperms) in lacking seeds. This classification divides ferns into four classes: Psilotopsida, Equisetopsida, Marattiopsida, Polypodiopsida. The last group includes most plants familiarly known as ferns.

Economic uses

Ferns are not as important economically as seed plants but have considerable importance. Ferns of the genus Azolla are very small, floating plants that do not look like ferns. Called mosquito fern, they are used as a biological fertilizer in the rice paddies of southeast Asia, taking advantage of their ability to fix nitrogen from the air into compounds that can then be used by other plants. A great many ferns are grown in horticulture as landscape plants, for cut foliage and as houseplants, especially the Boston fern (Nephrolepis exaltata). Several ferns are noxious weeds or invasive species, including Japanese climbing fern (Lygodium japonicum), mosquito fern and sensitive fern (Onoclea sensibilis). Giant water fern (Salvinia molesta) is one of the world's worst aquatic weeds. The important fossil fuel coal consists of the remains of primitive plants, including ferns.

Other ferns with some economic significance include:

Dryopteris filix-mas (male fern), used as a vermifuge
Rumohra adiantoides (floral fern), extensively used in the florist trade
Osmunda regalis (royal fern) and Osmunda cinnamomea (cinnamon fern), the root fiber being used horticulturally; the fiddleheads of O. cinnamomea are also used as a cooked vegetable Matteuccia struthiopteris (ostrich fern), the fiddleheads used as a cooked vegetable in North America Pteridium aquilinum (bracken), the fiddleheads used as a cooked vegetable in Japan and are believed to be responsible for the high rate of stomach cancer in Japan Diplazium esculentum (vegetable fern), a source of food for some native societies Pteris vittata (brake fern), used to absorb arsenic from the soil Polypodium glycyrrhiza (licorice fern), roots chewed for their pleasant flavor Tree ferns, used as building material in some tropical areas
Bracken fern, often poisons cattle and horses Cyathea cooperi (Australian tree fern), an important invasive species in Hawaii Dryopteris filix-mas, this fern accidentally sprouting in a bottle resulted in Nathaniel Bagshaw Ward's 1829 invention of the terrarium or Wardian case Ceratopteris richardii, a model plant for teaching and research, often called C-fern