An organism is a living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis. All organisms are composed of cells, which are the minimal functional unit of life. Organisms can be classified based on the number and types of cells they have as well as their feeding style. They can be unicellular, such as bacteria, archaea, and protists, or multicellular, such as animals and plants. Organisms can be further classified into prokaryotic and eukaryotic organisms. Prokaryotic organisms include archaea and bacteria, while eukaryotic organisms include all other organisms, such as animals, plants, fungi, and protists.

Living things can be identified and distinguished from inanimate objects by their distinctive characteristics. For example, an organism is made up of one or more cells, and it can maintain homeostasis, which is the ability to maintain a stable internal environment despite changes in the external environment.

The term "organism" first appeared in the English language in 1703 and took on its current definition by 1834. There is a long tradition of defining organisms as self-organizing beings, going back at least to Immanuel Kants 1790 Critique of Judgment. The scientific study of all organisms is called biology.