Organism

In biology and ecology, an organism (in Greek organon = instrument) is a living complex adaptive system of organs that influence each other in such a way that they function in some way as a stable whole.
An organism is in a non-equilibrium thermodynamic state, maintaining a homeostatic internal environment, and a continuous input of energy is required to maintain this state.
The origin of life and the relationships between its major lineages are controversial. Two main grades may be distinguished, the prokaryotes and eukaryotes. The prokaryotes are generally considered to represent two separate domains, called the Bacteria and Archaea, which are not closer to one another than to the eukaryotes. The gap between prokaryotes and eukaryotes is widely considered a major missing link in evolutionary history. Two eukaryotic organelles, namely mitochondria and chloroplasts, are generally considered to be derived from endosymbiotic bacteria.
The phrase complex organism describes any organism with more than one cell.