structural:expansion, information:filtering, structural:scholar_edition, linguistic:voice_change
Definition

There is not a single accepted definition of what protistss are. As a paraphyletic assemblage of diverse biological groups, they have historically been regarded as a catch-all taxon that includes any eukaryotic organism (i.e., living beings whose cells possess a nucleus) that is not an animal, a land plant or a dikaryon fungus. Because of this definition by exclusion, protists encompass almost all of the broad spectrum of biological characteristics expected in eukaryotes.

They are generally unicellular, microscopic eukaryotes. Some species can be purely phototrophic (generally called algae), or purely heterotrophic (traditionally called protozoa), but there is a wide range of mixotrophic protists which exhibit both phagotrophy and phototrophy together. They have different life cycles, trophic levels, modes of locomotion, and cellular structures. Some protists can be pathogens.

Examples of basic protist forms that do not represent evolutionary cohesive lineages include:

The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and plants or fungi (e.g. slime molds and flagellated algae like euglenids), have been published under either or both of the ICN and the ICZN codes.