1. Any of a group of eukaryotic organisms belonging to the kingdom Protista according to some widely used modern taxonomic systems. The protists include a variety of unicellular, coenocytic, colonial, and multicellular organisms, such as the protozoans, slime molds, brown algae, and red algae.
2. A unicellular protoctist in taxonomic systems in which the protoctists are considered to form a kingdom.