Unicellular
A unicellular organism is a living thing made of only one cell that carries out all life functions on its own, such as a bacterium or an amoeba.
Definition
A unicellular organism is a living thing made of just one cell, and that single cell has to do everything: eat, move, get rid of waste, and make copies of itself. It performs every function needed for life, including taking in nutrients, exchanging gases, removing wastes, responding to stimuli, and reproducing, and the group includes all prokaryotes (bacteria and archaea) plus many eukaryotes such as protists and yeasts. Because all metabolic, homeostatic, and reproductive work happens inside one cell, size is constrained by the surface-area-to-volume ratio needed for diffusion-based exchange. Unicellular organisms reproduce asexually by binary fission, budding, or spore formation, and many also exchange genetic material through conjugation or transformation. Some species, such as slime molds and choanoflagellates, form temporary colonies, which offers clues about how multicellularity arose.
Example
A drop of pond water can hold hundreds of unicellular organisms such as amoebas and paramecia, and bacteria and yeast are unicellular too. A single yeast cell about $5\ \mu\text{m}$ wide takes in sugar, ferments it into carbon dioxide and ethanol, and buds off a new cell every $90$ minutes or so, and the carbon dioxide it releases is what makes bread rise. Certain bacteria even count their population by quorum sensing: each cell secretes a signaling molecule, and when its concentration crosses a threshold the whole population switches on genes for bioluminescence, biofilm formation, or toxin release, as Vibrio fischeri does in the light organ of the bobtail squid.
Key Insight
Most living things on Earth are unicellular, and they were the only kind of life for most of the planet's history. Being one cell limits size but not success, because unicellular organisms reproduce fast and adapt quickly, which is why bacteria can develop antibiotic resistance within a few years. Unicellular does not mean solitary: biofilms, quorum sensing, and colonial protists show that cooperation among cells preceded true multicellularity by hundreds of millions of years.