An organism is a living individual — a discrete, self-maintaining system composed of one or more cells that carries out metabolism, responds to stimuli, grows, and reproduces. Single-celled organisms (bacteria, many protists) accomplish all of these functions within one cell. Multicellular organisms (plants, animals, fungi) are composed of differentiated cells organized into tissues and organs that divide labor among them.
The concept seems straightforward until you try to draw precise boundaries. Colonial organisms like siphonophores consist of physiologically integrated but genetically identical units (zooids) — is the colony one organism or many? Clonal plants like aspens can share a connected root system across thousands of stems — is the clone one organism? Lichens are composite systems of fungus and photosynthetic partner — one organism or two? The holobiont concept adds another layer: if a human depends on its gut microbiome for normal physiological function, where does the organism end?
These difficulties suggest that “organism” is not a precisely bounded natural category but a useful approximation that works well in many cases and breaks down at the edges. What makes something an organism is not a single criterion but a cluster of features — metabolic integration, reproductive unity, genetic identity, physical boundedness — that can come apart. The question “what is an individual organism?” is one of biology’s genuinely open problems, not a settled matter of definition.
Related terms
- Cell — the basic unit from which organisms are built
- Species — the category that groups organisms by shared ancestry and reproductive compatibility
- Metabolism — the chemical processes that keep organisms alive
- Homeostasis — the regulatory processes that maintain organismal stability