Protists are best defined as
Protists are best defined as predominantly unicellular eukaryotes that do not belong to plant, animal, or fungal kingdoms, making them a paraphyletic collection rather than monophyletic clade. They possess membrane-bound nucleus, mitochondria or reduced derivatives, and complex cytoskeleton, yet lack tissue differentiation characterizing multicellular kingdoms. Assemblage includes amoeboid, flagellated, ciliated, and algal forms across multiple supergroups. This negative definition reflects historical kingdom Protista concept before molecular phylogeny resolved them into diverse lineages such as SAR, Amoebozoa, and Excavata originating early in eukaryotic evolution.
Ref: Campbell Biology 12th ed., Chapter 28 Protists eukaryotes not plants animals fungi; NCERT Class 11 Chapter 2 Kingdom Protista definition