end(o)-

Internal; within.

Greek endon, within.

An endoscope is an instrument which can be introduced into the body to give a view of its internal parts; an endoskeleton is an internal skeleton, such as that of vertebrates; the endocardium (Greek kardia, heart) is the thin, smooth membrane which lines the inside of the chambers of the heart; the endometrium (Greek mētra, womb) is the mucous membrane lining the womb; endarterectomy (Greek ektomē, excision) is surgical removal of part of the inner lining of an artery.

The adjective endogenous refers to something having an internal cause or origin; endothermic to a reaction or process in chemistry accompanied by or requiring the absorption of heat; endocrine (Greek krinein, sift) refers to glands which secrete hormones or other products directly into the blood; endemic, restricted to a certain country or area, was formed in Greek (endēmios, native, based on dēmos, people).

Endo‑ is closely related to ento‑; its opposite is ecto‑ or exo‑.