- the internal skeleton of an animal, esp the bony or cartilaginous skeleton of vertebratesCompare exoskeleton
- An internal supporting skeleton, derived from the mesoderm, that is characteristic of vertebrates and certain invertebrates.
- The internal supporting framework of humans and other vertebrates, usually made of bone. Certain invertebrates, such as sponges and echinoderms, also have endoskeletons. Compare exoskeleton.