In biology, an organ or viscus is a collection of tissues joined in a structural unit to serve a common function. In anatomy, a viscus (/ˈvɪskəs/) is an internal organ, and viscera (/ˈvɪsərə/) is the plural form.

Organs are composed of main tissue, parenchyma, and "sporadic" tissues, stroma. The main tissue is that which is unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerves, blood vessels, and connective tissues. Functionally related organs often cooperate to form whole organ systems. Organs exist in all higher biological organisms, in particular they are not restricted to animals, but can also be identified in plants. In single-cell organisms like bacteria, the functional analogue of an organ is called organelle.