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 term 'pteridophyte' has traditionally been used to describe all seedless vascular plants, making it synonymous with 'ferns and fern allies'. This can be confusing since members of the fern phylum Pteridophyta are also sometimes referred to as pteridophytes.