Made of just one cell. All the living work — eating, growing, getting rid of waste, making copies of itself — happens inside that single cell, with no division of labour between parts. Bacteria are unicellular; so is yeast, and so is an amoeba. You and a tree and a fish are multicellular, made of billions of cells doing different jobs. This is a science word, at home in biology class, textbooks and lab reports, and almost nowhere else. In everyday English people say single-celled instead, which means exactly the same thing and sounds far more natural: "single-celled organisms". If you meet the word in an exam, though, unicellular is the term you want. A gentle warning about a habit of thought: unicellular does not mean simple or primitive. One cell can be extraordinarily complicated inside, and unicellular life has been succeeding on this planet far longer than we have.