The term protist is an informal term that encompasses multiple organisms from different common ancestors and life cycles.
The term protist itself is an informal term that encompasses multiple organisms from different common ancestors and life cycles. Despite greater understanding of cladistics and genetics, the term continues to be used informally.
The kingdom Protista is a diverse kingdom made up of organisms that are neither animals, plants, or fungi. Most members of this group are unicellular.
Imprimis, Carolus Linnaeus, in the 18th century, classified all living beings in two big kingdoms: Plantae and Animalia. In that moment, microscopic moving creatures were animals and the sessile were plants.
Later on, many classification systems emerged, and so did the distictions between Eukarya and Prokarya, as well as the propositions of more kingdoms, like Monera, Fungi, Protozoa (Proto = primary, Zoa = animal). Till the early 20th century, this vision of primitive animals still persisted.
More recently, studying the Eukarya domain, scientists noticed that Protozoa is a paraphyletic taxon (a taxon that does not include groups that are systematically inside it, like it happens with Reptiles, see: https://en.wikipedia.org/wiki/Paraphyly). And moreover, it is not placed in the right taxonomic level (kingdom), since there are many groups that are actually kingdoms inside the Protozoa "kingdom", so it should be a "superkingdom" or a "subdomain".
If you take a look at this page: http://tolweb.org/Eukaryotes/3, you will notice that there is no Protozoa kingdom anymore. Tecnically, Eukarya has 24 kingdoms. And more than 15 of them are what we could call Protists.