In biology, an organism is any contiguous living system (such as animal, fungus, micro-organism, or plant). In at least some form, all types of organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole.
An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell.
All organisms living on Earth are divided into the eukaryotes and prokaryotes based on the presence or absence of true nuclei in their cells. The prokaryotes represent two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus, and contain additional membrane-bound compartmentalization called organelles (such as mitochondria in animals and plastids in plants, both generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms that are eukaryotes. Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism.
The hierarchy of biological classification‘s eight major taxonomic ranks. Intermediate minor rankings are not shown.
All organisms are classified by the science of alpha taxonomy into either taxa or clades.
Taxa are ranked groups of organisms, which run from the general (domain) to the specific (species). A broad scheme of ranks in hierarchical order is: