unicellular organism
/ yoo-nih-SEL-yoo-ler OR-gan-iz-um /
A unicellular organism is exactly what it sounds like: an entire living thing made of just one cell. That single cell is not a fragment of something bigger — it is the whole organism. It must, on its own, do everything that staying alive requires: take in food, get rid of waste, sense its surroundings, move if it can, and reproduce.
Because one cell must run the whole show, a unicellular organism packs a surprising amount of capability into a tiny space. Bacteria and archaea are unicellular, and so are many eukaryotes — yeasts, amoebas, the green Euglena, and the malaria parasite. Some of these single eukaryotic cells are quite elaborate, with a nucleus, swimming tails, light-sensing spots, and tiny mouths. Reproduction is usually by one cell dividing into two, so a 'parent' and its 'offspring' are each complete organisms in their own right.
Unicellular life came first and still dominates the planet by sheer numbers and total mass. It shows that you do not need a body, a brain, or organs to be fully alive — a single well-run cell is enough. The contrast is the multicellular organism, where many cells specialize and depend on one another, none able to survive alone for long.
An amoeba in pond water is a complete animal-like life in one cell: it oozes along by changing its shape, surrounds and swallows smaller cells for food, and reproduces by splitting in two — no body required.
An amoeba: one cell that is also a whole organism.
Unicellular does not mean prokaryotic: many single-celled organisms (yeasts, amoebas) are eukaryotes with a full nucleus. The two distinctions are separate questions.