In biology, an organism is any contiguous living system, such as a vertebrate, insect, plant or bacterium. All known types of organism are capable of some degree of response to stimuli, reproduction, growth and development and self-regulation (homeostasis).
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.
An organism may be either a prokaryote or a eukaryote. Prokaryotes are represented by 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 compartments called organelles(such as mitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes.