An organ is a multicellular, multitissue part of an animal or plant, which forms a discrete structural and functional unit. (Tissues are formed of many cells that are similar in structure and function, and are bound together as a unit by intercellular material.)

Large, advanced animals have many kinds of organs. Some examples include: arteries and veins, bones, the brain, eyes, the heart, intestines, kidneys, the liver, ovaries and testes, the skin, and the stomach. Some of these are organized into functional complexes known as organ systems, such as the digestive system that is responsible for processing and absorbing nutrients from food, and in typical mammals is composed of the stomach, large and small intestine, and organs that synthesize and secrete digestive enzymes, such as the pancreas. Another example is the skeletal system, which is composed of numerous bones.

Plants also have many kinds of organs, such as leaves, stems, roots, and the various parts of flowers. An example of an organ system in a higher plant is a flower, which (depending on the species) may be composed of stamens (which are composed of anthers and a filament), pistils (containing stigma, style, and ovary), petals, bracts, and receptacle.