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 either be unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialised tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell.
The term “organism” (Greek ὀργανισμός – organismos, from Ancient Greek ὄργανον – organon, “instrument, implement, tool, organ of sense or apprehension”) first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary). It is directly related to the term “organisation”. There is a long tradition of defining organisms as self-organising beings, i.e.; any complex thing or system having properties and functions determined not only by the properties and relations of its individual parts, but by the character of the whole that they compose and by the relations of the parts to the whole.