A phylum (plural: phyla) is the third highest rank used in the biological taxonomy of all organisms. The second rank is kingdom and the highest is domain. Usually biologists count about 32 phyla of animals, about 14 of plants, and about 8 phyla of fungus, but definitions vary.

With genome analysis, groups of phyla have been put together based on evolutionary relationships. These are informal, not part of the standard classification.

The exact definition of a phylum is subject to debate, but most often it is either taken to mean a clade where a basic body plan/body structure is present in all its members or to mean a "grand"/"basic" grouping of life in which subsequent extinction has led to its representing a great amount of biodiversity, particularly because it is rare to find lifeforms not in any phylum (although this may be circular logic) except in the fossil record.

Botanists usually prefer the word division instead of phylum.

• List of animal phyla