An organism refers to a living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis. An organism would, therefore, be any animal, plant, fungus, protist, bacterium, or archaeon on earth. These organisms may be classified in various ways. One of the ways is by basing upon the number of cells that make it up. The two major groups are the single-celled (e.g. bacteria, archaea, and protists) and the multicellular (animals and plants).
