Endo

Endo is a combining form or prefix used in science and medicine to indicate internal location, inner parts, or internal origin. It derives from the Greek endon, meaning within.

In anatomy and physiology, endo- appears in terms such as endocardium (the inner lining of the heart), endothelium (the inner lining of blood vessels), endometrium (the lining of the uterus), and endoderm (the innermost of the three germ layers). In cell biology, endoplasmic reticulum and endosome are intracellular compartments, and endocytosis describes the process of bringing materials into a cell. Endoskeleton refers to an internal skeleton in some animals, while endosymbiosis describes a theory that certain organelles originated from intracellular mutualists.

In chemistry and physics, endothermic systems absorb heat from their surroundings. In physiology, psychology, and related fields, endogenous refers to processes, substances, or factors that originate within an organism or system, whereas exogenous refers to those from outside.

Geology and earth science also use endogenic (or endogenous) to denote processes that originate within the Earth, such as volcanism and tectonic activity, in contrast to exogenic surface processes like weathering and erosion.

In medical and biological terminology, endo also appears in terms such as endocrine, where secretion occurs within the body, illustrating the broader sense of within or internal activity. The prefix is widely used across disciplines to denote internality, often paired with clear antonyms such as exogenous or external.