The prefix [endo-] is of Greek origin and means "inner or within". There are many uses of the term as follows:

Endocardium: the root term [card] means "heart" and the suffix    [-ium] refers to a "layer or membrane" - Inner layer of the heart
Endocrine: the suffix [-crine] means "secretion", the word meaning "inner secretion". Refers to a gland that deposits its secretions within the bloodstream. The products of endocrine glands are known generically as "hormones"
Endometrium: the root term [-metr-] is Greek, meaning "uterus" . The word endometrium means "inner layer of the uterus"
Endoscope: the term [-scope] refers to an instrument used for viewing. There is a consensus that a viewing instrument that enters through a natural body cavity will be called an "endoscope" (see image). All others will adopt the name of the cavity that is being viewed, as a laparoscope, a thoracoscope, an arthroscope, etc.

Original image courtesy of  Wikipedia.