Protist
A protist is a eukaryotic organism, usually single-celled, that is not a plant, animal, or fungus, such as an amoeba, paramecium, or alga.
Definition
A protist is a living thing, usually a single cell with a nucleus, that is not a plant, animal, or fungus, and most live in water, from an amoeba oozing along to wrap around its food, to a paramecium zipping through pond water with tiny hairs, to green algae floating in sunlight making their own food; protists are really an "everything else" group, gathered together mainly because they fit nowhere else rather than because they resemble each other. More precisely, protists are eukaryotic organisms outside the plant, animal, and fungal kingdoms, mostly unicellular though some like kelp are multicellular, informally grouped by how they get food and move: animal-like protozoa that eat other organisms, plant-like algae that photosynthesize, and fungus-like slime molds that absorb decaying matter, with some, such as Plasmodium, causing disease. Technically, protist is a paraphyletic term spanning most major eukaryotic supergroups, including SAR (stramenopiles such as diatoms and kelp, alveolates such as ciliates and Plasmodium, and rhizarians), Excavata, Amoebozoa, and unicellular members of Archaeplastida and Opisthokonta, and many protist lineages acquired chloroplasts by secondary endosymbiosis of a eukaryotic alga, producing plastids with three or four membranes, while choanoflagellates are the closest living relatives of animals.
Example
Plasmodium, the protist that causes malaria, is carried by mosquitoes and infects red blood cells, killing several hundred thousand people yearly, and Plasmodium falciparum still carries a relict plastid called the apicoplast, evidence its ancestors were photosynthetic and now a drug target because human cells lack it. Diatoms, tiny algae with glass-like shells, fix about $20$ percent of global carbon each year, comparable to all the tropical rainforests on land, and a paramecium packs a mouth, a way to expel excess water, and thousands of moving cilia into a single cell.
Key Insight
Protists are where the history of the eukaryotic cell is written, since the endosymbiotic events that produced mitochondria and chloroplasts, and the origins of multicellularity in animals and plants, are best understood by studying them.