In biology, an organism is a living thing (such as animal, plant, fungus, or micro-organism). In at least some form, all organisms are capable of reacting to stimuli, reproduction, growth and maintenance as a stable whole (after FAO[http://www.fao.org/biotech/find-formalpha-n.asp Biotechnology in Food and Agriculture ] ). An organism may be unicellular or made up, as in humans, of many billions of cells grouped into specialized tissues and organs. The phrase complex organism describes any organism with more than one cell.

The term "organism" (Greek ὀργανισμός - organismos, from Ancient Greek ὄργανον - organon "organ, instrument, tool") first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary).

Organisms may be divided into the prokaryotic and eukaryotic groups. The prokaryotes represent two separate domains, the Bacteria and Archaea.T.Cavalier-Smith (1987) The origin of eukaryote and archaebacterial cells, Annals of the New York Academy of Sciences 503, 17–54 All fungi, animals and plants are eukaryotes.

The word "organism" may broadly be defined as an assembly of molecules that function as a more or less stable whole and has the properties of life. However, many sources propose definitions that exclude viruses and theoretically-possible man-made non-organic life forms. Viruses are dependent on the biochemical machinery of a host cell for reproduction.

Chambers Online Reference provides a broad definition: "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction" .

In multicellular life the word "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory, digestive, or reproductive) themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent.

Viruses

Viruses are not typically considered to be organisms because they are incapable of "independent" reproduction or metabolism. This controversy is problematic, though, since some parasites and endosymbionts are also incapable of independent life. Although viruses have a few enzymes and molecules characteristic of living organisms, they are incapable of reproducing outside a host cell and most of their metabolic processes require a host and its 'genetic machinery' such as organelles in eukaryotic hosts and the assemblage of ready-made enzymes (which the virus cannot make by itself) in prokaryotic hosts. While viruses sustain no independent metabolism, and thus are usually not accounted organisms, they do have their own genes and they do evolve by the same mechanisms by which organisms evolve.