An organism may be either a prokaryote or a eukaryote. Prokaryotes are represented by two separate domains—bacteria and archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles. Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism. An organism is an individual entity that exhibits the properties of life, including reproduction, growth and development, maintenance, and response to stimuli. All organisms are composed of one or more cells.

A material entity that is an individual living system, such as animal, plant, bacteria or virus, that is capable of replicating or reproducing, growth and maintenance in the right environment. An organism may be unicellular or made up, like humans, of many billions of cells divided into specialized tissues and organs.