Definition of PROTIST. : any of a diverse taxonomic group and especially a kingdom (Protista syn. Protoctista) of eukaryotic organisms that are unicellular and sometimes colonial or less often multicellular and that typically include the protozoans, most algae, and often some fungi (as slime molds).— pro·tis·tan \prō-ˈtis-tən\ adjective or noun. See protist defined for kids.ADVERTISEMENT.rotoctista) of eukaryotic organisms that are unicellular and sometimes colonial or less often multicellular and that typically include the protozoans, most algae, and often some fungi (as slime molds). — pro·tis·tan \prō-ˈtis-tən\ adjective or noun. See protist defined for kids. ADVERTISEMENT.